uv protection would be one guess.
The others are:
a) possible extended dosing evaluation-- recall, Dey/Mylan doesn't exactly have a profit motive for testing efficacy/degradation out past expiry. My guess is that they haven't. There's just not much literature on the subject either way-- and yet people routinely store them in high heat and then report some efficacy even with devices which are YEARS out of date, so...
b) proprietary buffering with redox scavengers-- but this one is something of a long shot, IMO. They might degass buffering solutions with nitrogen or even with a noble gas, though-- and that might really make a HUGE difference when coupled with excellent UV protection.