It's not as if every single K-pop fan altogether unanimously decided what the end and start times are for each gen; there's still a lot of discussion about it.
Nor is it the case that the gen start and end times are the same for boy groups and girl groups.
I think the overview below was the most accepted classification for gen determination:
As you can see, for boy groups it is said that 3rd gen started with EXO in 2012, while for girl groups it started in 2014 with Mamamoo and Red Velvet.
I think that maybe the 3rd gen era seemed so short especially for girl groups, is that in 3rd gen there were only a handful girl groups that were successful: you had BP, TWICE, RV, Mamamoo, GFriend and after that there was this huge gap where other girl groups struggled to survive.
When the 4th gen (girl) groups started to arrive, they blew easily past most of those struggling 3rd gen groups, which might be the reason that K-pop fans saw them as a new wave incoming: the 4th gen.
And in 4th gen there are more than just a few groups that are really doing well.
Note: I think that generational changes were also slower in earlier gens, because K-pop was more localised, with fewer active groups and as a result also changes and trends being slower.
