well, you got me curious. Took the bike out and mucked with the derailleur a bit. It only moves less than 1mm upward when i shift. The extra LX derailleur i have sitting on my desk moves upward a lot at the end of its throw, but the travel near the bottom, where it actually shifts, is flat. An old acera derailleur i found in the parts box moves upward a whole lot right from the start, as does the shimano 600 one on my roadie (which has much less curve and has to move up to reach both chainrings).
This leads me to conclude that the derailleur doesn't have 1 specific point where the chain is supposed to line up but a larger area, except maybe on the acera or 600 derailleurs. I think that it's less noticeable that the chain is hitting the 'correct' part of the derailleur than it is that the chain is shifted closer to the top of the cage, where its stiffer.
Whatever, i know what works better on my bike.