blue portals exp is random, what else is random?

It says that portals give 5 experience, but I have been testing it for a while now by opening one portal then extracting, to two portals then three portals etc. Checking the amount of exp after each extraction, so far it has been random:

one blue portal = 5-8 exp (two times tested) two blue portals = 10 exp (two times tested) three blue portals = 15-18-33 exp (three times tested)