Quoting myself from a Deskthority thread:
The hybrid profile is better because there is an increased vertical step between the home row and each row further back. Here’s a diagram showing a bit better.

(note this shows SA row 2 on the number row, rather than the SA row 3 shown above, and a non-flipped DSS row 1 for the Q row; I need to test some of these in person to decide exactly what set works best)
The idea is, as you unbend your finger (extend the two distal joints) to reach further-away rows, the fingertip naturally travels in a direction upward and outward. Therefore, having the further keys be at the same height as the home row keys requires you to reach down by either moving your hand or flexing your first finger joint, the former of which is slow especially if you need to type keys on different rows with the same hand and the latter of which reduces the strength available for pressing the key and I suspect increases fatigue for keys with long travel. (It would be nice to actually have some proper scientific evidence here, unfortunately all the studies I’ve seen on related subjects are pretty crappy.)
Anyway, by raising the height of the further away keys, you make them easier to reach. The closer rows, by contrast, are reached by flexing mainly the second finger joint, and I don’t think they benefit much from being lower. (Though again it would be nice to see some comprehensive empirical testing.)