Their verbal abilities, where they shine, are probably due to the intensive one-on-one coverage they have had from day one from Mom and Nana and Dad and (belatedly) Gege.  People thought we were devoting way too much adult-time to raising two kids, lavishing too much time and attention and care on them.  Maybe not.

Maybe the attention lets the kids  appear verbally bright  now, just as giving them the sign language made them seem unusually bright a year ago.

A year ago they were doing what most kids could probably do if someone took the time with them.  Today's verbosity reflects Nana's patience repeating things over and over and sitting through more repetitions of Blue meets Spot than seemed necessary
at the time.
Maybe it's the caregivers that were bright and the kids just kids.  And maybe we've all developed higher expectations that are reasonable:

I went over to the King's at 2 yesterday afternoon to put together trikes for their birthday.  By 3:30 it was obvious I was usurping too much of Alex's "father fun"; the self-assembly stuff.

At 3:45 the twins got to try the trikes.

By 4:05 the newly assembled and reassembled to get it right trikes were hidden away in the garage, where they will stay for the next six months.   The little girls are too short to reach the pedals and too young to have enough arm strength to turn the handlebars if someone is pushing them, and the trikes are too big for them to maneuver around the corners in the house and we didn't disassemble the damned things because of the fun being too much.

Dr. Mom jumped the gun by suggesting we get them the trikes and we went along with it.  Box clearly said Ages 3 and up.   Verbal preciosity does not imply equal physical ability.

When asked "How old are you?" both reply, "Two.  Years.  Old,"  solemnly and with head nods on each word.  See, they know they are only two -- it's the parents and grandparents that tend to forget.