*facepalm*
It's because they streamlined the Macc formula for iLvs.
When you have an iLv weapon equipped, your Macc is determined by the "fake" skill of the weapon, in part.
Before this update, there was not a "help text" for this... And on many iLv staves that had +X Macc before the streamlining of the formula... upon the update that streamlined the formula, they "loss" their Macc bonuses...
In reality they either maintained or gained a substantial Macc bonus.
So in THIS update....
They added another "fake" skill stat to ALL iLv gear.
How you are to determine the exact Macc from the "Magic Accuracy Skill" is to use your normal Macc formulas:
M=Magic skill + M1 + M2
Where M2 is your total magic accuracy from gear/merits and M1 is the contribution of your stats to magic accuracy:
δstat=(caster's stat)-(target's stat) M1
δstat ≤ 10 M1 = δstat
δstat≥10 M1=10+(δstat-10)/2
And increase w/e skill by the listed amount.
So for Macc for an elemental spell... you use:
Macc = elemental skill + "Magic Accuracy Skill" + dINTmod + Macc from gear/merits/wherever else
This was streamlined because "hybrid" jobs like DRK, RDM, or BLU were having Macc problems at iLvs...
So their "fake skill" was used in some unknown fashion...
And the Devs just got tired of dumping massive ammounts of Macc stats on iLv staves and clubs... so they merged it all together and now all jobs use the "fake" skill stats...
And then in this update they added the "fake" Macc skill stat so we wouldn't have to figure out what X "fake" combat skill translated to for "fake" Macc skill.
/history lesson.