Uuuuuuh guys?

Base combo rate is 8% + 3%, right?
Tiara Set Bonus is 15% + 15%, right?
100 Double Union Grail buff is 25%, right?
Tiamat is about 5%, right?
So I should have... about 71% Combo, right...? So how come I have more like 90%+?

I had to watch like four AABs before to confirm that I actually CAN miss a Combo attack. It should be noted that my Water Grid is starting to be an antique by now. I'm still playing the catch-up game from quitting for four months, focusing on modernizing one element at a time for each Tower. All I have 30 slvl of Triple in one Sloth Glaive - no Phantom Glaive or anything. According to DMM wiki, that's 1.5% more combo. I'm not running Shingen nor Aphrodite either.

Something is amiss, and I'd very much like to know what exactly it is.