The differential approach is said to enable a more consistent switching point, allowing the amplifier to be linear over a larger part of its range. It's probably true, but at the expense of added area and power consumption. Also, the PREAMP block should have an wide common mode range and good high-frequency common mode rejection, which might increase power even more.
Altasens' application US20080316342 looks quite obvious to me. It talks about gain control with fine steps. The gain control is split between analog and digital. The analog gain provides coarse gain steps and the digital gain provide finer gain steps between adjacent coarse analog gain values. Since analog gain can suffer from low precision, on-chip calibration is implemented to calibrate the analog and digital gain.
If granted, it would become quite broad and strong patent, but to me it seems obvious and I think there would be some prior art, even though I can not point to it specifically.