When you unrack the bar, there's a range of a few inches inches where the bar is going to feel lighter than if it were closer to your head or feet, right?
This is where you want to be when you start. You bring it down to the top of your sternum and bring it back to the sweet spot. For many people that's back slightly towards the head. If you try bringing it straight up from there, your arms at lockout will extend further up than if your had followed the more natural curve up (adding inches you'd then have to press) besides possibly giving yourself disadvantageous leverage. Some benchers with severe arches do indeed end up bringing the bar almost straight up. On decline, of course, you've change the angle of leverage. Mine moves away slightly then,also.
Hope this makes sense.