Thats really weird, because i am pretty sure my fps dropped to 30 everytime i started recording with fraps and same was for selda. Maybe they changed something, but i am positive this used to happen. Talking about 'trial' version, without any advanced options. Tho haven't used fraps for about 2-3 years.
EDIT:Just tried it out and you are right, it records in multiples of fps. Altho there is option to lock the fps, its still just an option. I guess they must have added it, but i really think that the old fraps had locked fps by default without any option to disable it.
EDIT2:Yup, fraps 2.2.4 had no option like that