Member exclusive fare from YVR to CUN seat selection fee

When booking in the description it said that seat selection for a fee.
But when I tried to select seats, it only showed fee for the seats in the front of the aircraft.

about like 50% of the seats were no fee to select?

Is that normal?

edit: I figured out why, It is because I am booking with a kid, so all of us gets free STANDARD seat selection regardless of type of fare we got.