10-8 wouldn't be .500. Just saying.
Yes, UCLA played well at home. Let's just assume if you played in the Big 12 you would also win all your home games.
But... UCLA hasn't been great on the road. In conference, you only played two top-50 (Pomeroy) teams in Arizona (11) and USC (34). You lost both of those games. And you barely won by 1 at WSU (54) and by 4 at Colorado (68). Not to mention getting beat by Baylor (9) and Illinois (28) at neutral sites.
If we extend that to the Big 12, and assume road losses to everyone better than USC, that means you get losses at: Texas (8), KU (9), Baylor (14), WV (17), KSU (18), ISU (22), TCU (25)
It is also reasonable to think that one of OSU (40), OU (51) or Tech (53) would get you at home since they are all better than WSU who you squeaked by.
Boom. There are 8 losses. And that assumes you go undefeated at home, something KU, Texas and Baylor were all unable to do.