I noticed that many games (if not most or all) are employing different leaderboards for different platforms. For example, different leaderboard for iOS and Android. Different leaderboard for Xbox and PS.
I can't see the logic behind that. After all, let's say iPhone and Android are sending the data to a single service/system.
Is it a business decision?
Different platforms may result in different scores even with the same skills. For example because you were forced to have less enemies on the Android game due to performance issues.
This (along with the control difference) is why console and PC games generally never play together (either multiplayer or through leaderboards).
It might be a business decision. It might be a convenience decision. It really depends; the only thing you can say for sure is that it's the decision of the people involved in making the game (developer, publisher, platform vendor, et cetera).
You're assuming that "iPhone and Android are sending the data to a single service," which isn't always the case. The iOS APIs for leaderboards (Game Center) send data to Apple. Google's Leadersboards service sends data to Google. A developer may choose to use the platform-provided leaderboard API for his or her game because it's easier or cheaper for them (despite the fact that they'll need to build an implementation against both APIs). They may also choose to do it because those APIs offer better device integration than 3rd party APIs might, and they feel this creates a better experience for the player.
There are various third-party leaderboard APIs a game might adopt. Some of these may have additional cost (either as retail products or in the form of requiring the developer to provide and pay for the hosting of the leaderboard databases). They may not look or feel as integrated with the device, either. Those potential disadvantages have to be weighed against the advantage of only having to write against the API once (in theory), reducing development time.
The particular balance of all those factors (and more) will all play into the decision of which particular method to use for leaderboards; it basically depends on the developer's needs and wants.
If you use GameCenter's built-in leaderboard functionality to avoid having to implement leaderboards yourself, then you end up with seperate leaderboards when you make your Android version.
For me, it's a decision that goes like this: "I don't want to reimplement a leaderboard server just so I can have cross-platform leaderboards."
Most of the answers here cover the reasons why.I would like to mention when it comes to iOS and Android development a new situation arises.
Apple favors games and apps that use native iOS features. You are more likely to be featured if you are using Game Center for leaderboards. They highly recommend it and may even contact you with offers to promote you on the front page if you switch to it.
Android is the same way with Google Play and Google+. So having two separate leader boards a GameCenter and GooglePlay/Google+ version is highly advantageous to get featured and promote your game.
New member, can't comment but I agree with these answers. As a gamer I would like to see my "global" rank on a game that is cross platform. However as a Software Developer I understand why they are separate.
The first thing that comes to mind is the fact that they are different companies (what @JoshPetrie mentioned). Microsoft has their own servers and databases while Sony has theirs. Since they are separate companies with separate data hubs, there will naturally be separate rankings. They would have to work together and share a system that houses both datasets. That doesn't seem likely because that would mean that Microsoft has access to Sony's data and vice versa. And nothing is more personal to a company than their data.
Let's assume however that this were true and there was a shared database where all the rankings are together. I bet the rankings wouldn't be as evenly distributed as one might think and it would be due to hardware differences which result in a big difference in the ability for a user to master the game. Each gaming console is a different environment, or medium, in which the user can play the game. Based on its design, the environment creates limitations on "how well" the user can play with respect to users on a different medium.
The biggest example I can think of (and was already mentioned in another answer) is the difference between Console and PC.
Let's use Team Fortress 2 as an example. The console version is played with a hand held controller. Usually the contoller is operated with both thumbs and index fingers (4 digits, at least how I play). The PC version uses the keyboard with W-A-S-D movement, other keys for functions, and the mouse (6 digits used at once, by my count).
Not only are the "controllers" different, but, like @ratchetfreak said, each system has its own degree of performance power. So the exact same game could be slow and laggy on Xbox, but very fluid on PS or PC.
Also PC's can be upgraded as opposed to console games which are static until the new version comes out. Which then the leaderboards would be different since a newer version of the game is required (which means a different server and database and blah blah...)
On top of these differences, you also have system settings like mouse speed which can be increased higher than joystick sensitivity (which, for me at least, makes up the biggest advantage of PC gaming).
Many other factors are involved I'm sure but these were the first things that came to mind when I saw this question. Sorry for being long winded, I got excited and blurbed it out as fast as I could.
