Ah, you're right, that must be it. It's quite subtle, but certainly a noticeable change for someone who looks at iOS icons more often than I do.
The idea with that bevel is probably to make the icons look somewhat more three-dimensional when they are "stood up" on a perspective plane (like the OS X dock does with icons in its default position at the bottom). The problem of course is that the icons were not designed for this kind of pseudo-3D look, and that's not something that can be fixed by simply compositing a sharp dark shadow at the bottom edge of the icon.
The idea with that bevel is probably to make the icons look somewhat more three-dimensional when they are "stood up" on a perspective plane (like the OS X dock does with icons in its default position at the bottom). The problem of course is that the icons were not designed for this kind of pseudo-3D look, and that's not something that can be fixed by simply compositing a sharp dark shadow at the bottom edge of the icon.