Both the AI and B-spec drivers don't like to pit until their tires are really bad... so on some tracks, where the pit entrance comes right after a turn, they can miss it due to the lack of grip, and will have to go another lap.  On a long track like Sarthe, that can spell disaster as not only will they have no grip, but they can also run out of fuel. 👎
So when you are using B-spec, its a good idea to plan your pit stops ahead of time, and not wait for the B-spec driver to pit on his own.  This will also save you the 5 second wait time added to each of your pits when you don't respond to the pit command.