Ammo is never given to you for free. The only reason why it appears that you got ammo back is because after 30 seconds, the game searches for nearby ammo drops and gives them to you. So if you get nothing, then there's no ammo on the ground.

You can test this yourself by using all your bullets and staring at a purple ammo drop a few meters away. It'll disappear after 30 seconds, and you'll have heavy ammo again.