There's a lot that goes on when you release the arrow. I'll bet that if you had one of those ultra-fast videos of your arrow, it would wiggle like a piece of spagetti when you shot it, even though you have it lined up as close to center-shot as you can get it.
One of the things that goes on when you shoot with fingers is that the string rolling off your fingers throws the rear of the arrow to the left (for a RH shooter). This is one of the reasons you really don't want your arrow completely center-shot for a finger release. Having the front of the arrow a little left of center helps to offset the leftward movement of the nock.
There are a lot of other things that go on when you release the arrow. Fortunately, you don't have to know what all of them are, because when you bare shaft tune you can deal with results and not causes.