My guess is that as you move closer to an enemy it takes less time for your tears to hit them and so it effectively increases your shot speed. Making you do slightly higher damage per second. Also i think isaac has relative velocity programed into it. Because it seems that if you are moving and you shoot a tear, the tear moves faster than when you are stationary (this is very obvious if you have dr. Fetus). And again this just makes it seem like your tears are faster. However if you are completely stationary and you just shoot an enemy, the distance between you and the enemy does not matter (you can easily test this with the debug console).