In lots of WebGL online games (all games I tested were made with Unity), my FPS is sometimes higher than the max, FPS setting of my PC (75), but why?
There are multiple rates being measured here.
The game is telling you how many times it's redrawing the scene with a frame that could be displayed.
Every 1/75th of a second, your computer refreshes your screen and displays the most recent frame.
If the game were running at 150 FPS and your screen at 75Hz, you'd be seeing roughly every other frame generated by the game.
This disconnect happens because WebGL (or other window-mode games) don't take full control of the display, so the OS is still responsible for redrawing the screen as a whole. It then uses one of several methods to render the game content (sometimes it uses a buffer and treats it like any other window content, other times it offloads to the GPU for the render directly).
Which method it uses depends how the game's been written, what the GPU and OS support, whether hardware acceleration is used, etc, etc.