My theory as to why you have to wait longer to punish some moves (more or less blockstun) is because NRS is focused on the animations of each move and then makes the block stun to give whatever frame data they wish. In other games there's one set amount of blockstun for every move (or every hit strength) and then they tailor the recovery of the move to match that blockstun.
for example, I made up these numbers
NRS - Shao Kahn Shoulder has 15 frames of recovery and they want it -1, so it has 14 frames of blockstun
Street fighter - Ryu uppercut has 5 frames of block stun and they want it -15, so it has 10 frames of recovery
same variables, but a different relationships and priorities