A look at the rosters of the Belsize Stros and Arlie Latham shows that both teams field Excellent and Very Good defenders wherever they can; those two teams are by far the two best at converting balls in play into outs. The overall league IPavg (found on the team pitching leaderboard) is .292, with the rest of us allowing batting averages on balls in play from .276-.316. The Lathams meanwhile hold opponents to a .262 average on balls in play, while the Stros hold opponents to a .249 average on balls in play.
The Stros have also committed the fewest errors in the league (38 through 109 games), while the Lathams are 9th in errors with 56, a very solid showing.
So if it feels like your hitters suddenly struggle against those teams, it might be because hits got changed into outs by the defense. Neat!