Dont you just love it when 1 simple comment causes half a dozen posts just to clarify something
Im pretty sure the rule stands that you must make what LFS classifies as a pit stop, im guessing 'Must Pit' is on, so each driver will have to stop and wait while the pitstop is carried out doing whatever you ask it to do (oh, plus those insignificant damage repairs that take 2hrs just to knock out a dent on the fender!) and then your pit stop is complete.
Unless the rule stands as 'MUST add >5% during the pit-stop' which would be impossible to regulate, im fairly confident that that its just a case of 'Must Pit' being enabled.