Neither. The authors and artists do. It has nothing to do with game mechanics, but is a function of the game world design and its presentation.
option 1: let's magically look at your stats and decide if you can join us or not
option 2: let's allow you to join us, but not to get into our institute of higher learning until you have proven your worth and are introduced to the schools of magic in their most basic form in your guided tour of Tamriel and basic instructions in the magical arts
option 3: let's allow you to join us because we're always looking for more recruits and we're low on recruits, so we're hiring again; if you get the job done(basically as part of the group of law-abiding mercenaries), we don't care about invisible numbers
option 4: murder someone, and we will contact you; murder another person to prove your dedication and ability to murder, and we don't care for imaginary numbers, we care for action
option 5: steal, and word may reach our ears if you are caught, or word may reach our ears of your snooping around for information about us; we will contact you and allow you to join our petty group of thieves who merely steal for a living and aren't too picky on imaginary numbers if you pass a test to prove your worth
option 6: ha ha ha ha, you won't last, but if you're serious, sure, we could use another suicidal idiot to die in the arena; I didn't expect you to succeed, but since you did, you're good enough to fight here, screw imaginary numbers
Which options are more believable and which one is the one used by Morrowind while the others are used by Oblivion?