I think there were more Yeerk pools, at least around the USA, even if they must be smaller. More we approach to the end of Animorphs, more the Animorphs should travel to fight in other cities, other states...
I also felt it strange when I saw Visser 3 was so often in this city. But maybe he had other Vissers or sub-Vissers for the other cities/states/countries.
For the RPG I gonna make, I'll try to fix it, with more pools around Earth and Vissers or sub-Visser for each one.