The difference is that, unless you are riding no hands ALL the time, some of your weight is forward of the cranks, which means that the amount of weight behind them determines where the point of balance over them is (while pedaling with any real force). Try this: stand straight up, lean forward at the waist, see how far you can lean forward. Now stick your butt out and try the same thing.

I've helped hundreds of people with riding pain while working at a shop, and this is a pretty typical issue. People move their seat forward to reduce the reach to the handlebars, and make things worse by shifting their balance over their hands more. You CAN compensate by moving the seat around on the post, but only to a point. The angle of the seat tube determines the range you can work with.