Is that an error on the game? Just because you CAN do more than the minimum (which is 4 SP, 4 RP and later increased to a soft "10"), does that mean that if they don't carry more, then they are doing something wrong? Is it actually a thing to where "You can not have less than 50% of your active roster be pitchers"?
I would CERTAINLY bring in 15 position players and 11 pitchers almost every year. (But I am not sold on this "5 man rotation is not enough" thing. Baseball functioned excellently with a 4 man rotation until the mid 90s, a 3 man rotation until the 60s, etc...)
I question if the AI having a smaller pitching staff is a problem. What is the official MLB ruling on "the 26th man"? I know the rule prohibits more than 13 pitchers, but it absolutely does not say you are not allowed to have LESS than 13...
Edit:
I know this is not necessarily your issue. Your league settings say 13/13. But that is not a mandatory hard rule, as far as the settings are concerned, although it looks like it should be an exact number as such. I do believe that setting allows for a variation, probably +/-2. And if I'm wrong, ok!
I'm only replying based on the "BCS rule".