As we know, some parts have been redesigned by TLG over time, and this is also the case of many Technic parts.
Sometimes existing parts are discontinued, others replaced by new alternative designs, but in some cases certain parts receive just some small design changes. Either to strengthen them, to facilitate the production process and/or reduce production costs.
Several Technic gears have suffered redesigns in the past and it was now also the case from Z16 gea.
According to the BrickLink database, it seems the change was first noticed within set 8043, whose first boxes were shipped with the previous Z16 version (4019) and are now being shipped already with the correspondent new design (94925).
I don't know exactly what motivated this change, or which problems the previous design had that lead into this modification, but guess that the strengthen was the purpose. The fact that the previous design didn't have a closed surface immediately close to the place where to insert the Technic axles, might have been the issue to address.