I'd assume it's that they just decided to throw Adv Mastery in random places instead of balancing out skill bonuses, then there's jobs with a specified Adv Mastery skill.
Adv Combo itself is already a strong skill prior to the mastery bonus. Seeing as Combat Mastery got increased to a 30 SP skill and gives 50% PDR only, it doesn't feel worth the 30 SP. They should have threw the mastery bonus onto Combat Mastery rather than making Adv Combo an even more worthy skill.
The same goes for Arch Mages, they threw the mastery bonus onto summons when they could have lowered the summons to 20 SP and 1 other skill from 30 to 20 SP. Then from there, make Buff Mastery (Magic Master) to 30 SP giving the bonuses of 70% Mastery, 30 MA, 50% buff duration.
There are plenty of skills that aren't worth the amount of SP they cost, and probably a few that are worth more than they cost.
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