You are right, actually. I blindly trusted the computer to figure this one out, but just looking at it I found a better solution by hand. Maybe there really are some better solutions for the 22 monitors problem.

spoiler

This better not end up in any scientific papers, ever.

To answer your question: fitting more monitors into these gaps would not result in the optimal placement for that amount of monitors. 23, 24, and 25 monitors all have "expected" results. And closing the remaining gap would not result in a smaller bounding box square.