its like anything else you buy as a consumer.
$$$ (profit) drives alot of decisions when it comes to consumer goods. And saying (marketing) your gun can carry X many (more) bullets than your competitor maybe a bigger draw for those new to guns.
also, there may have been technological changes/improvements over time that would allow things to improve. Such as squeaking out 1 more MPG out of a car. IIRC, some of the subarus put in active shutters in the front grill to close at x speed. It suppose to give you 1? more MPG.
CZ may have redesigned the follower and/or spring to make that possible. who knows if it was on the "to do list" for years, but to me, it doesnt matter.
Also, there will always be someone where 1000 round magazine or a nuclear bomb wont be enough.