Why does OnTriggerEnter2D/OnCollisionEnter2D work when there is no other collider object opposite?

Question

my bullet:

1. There is a Box collider (checked "is Trigger")
2. It has Rigidbody2D (Gravity = 0, freeze rotation Z)

https://i.sstatic.net/ik1IY.jpg

I added the "OnTriggerEnter2D" function because isTrigger is checked. Bullet script:

private float lifeTime = 1.0f;

private void Start()
{
    Destroy(gameObject, lifeTime);
}

private void OnTriggerEnter2D(Collider2D collision)
{
    GameObject effect = Instantiate(hitEffect, transform.position, Quaternion.identity);
    Destroy(effect, 0.2f);
    Destroy(gameObject);
}

When I fire, the OnTriggerEnter2D function works even though there is no collider/object in the direction of the bullet. It only works a certain distance away from my main character, and it's always the same no matter which direction I shoot.

Answer 1

Here's how I fixed the inside of my OnTriggerEnter2D function:

if (collision.tag == "Enemy")
    {
        GameObject effect = Instantiate(hitEffect, transform.position, Quaternion.identity);
        Destroy(effect, 0.2f);
        Destroy(gameObject);
    }

The problem is fixed, but why do I have to do an "if" check? Doesn't the trigger function already check this?

If it only touches things with tags "Enemy" and colliders... I guess I'll have to add this for many different objects?

EDIT: I have a pistol collider in the position where the bullet was created, I made the pistol collider a little smaller and the problem went away. However, I adjusted this so that the colliders of the pistol and the bullet do not intersect, and it looked like that on the scene screen. Also, I couldn't quite understand the problem since the projectile was destroyed at a much greater distance from my pistol.