Studio Gobli appears to disagree with quite a few of our Mythic WS mods, even including easy shit like Magic WSs.
The thread where we tested the Mythic WS properties is huge and full of people that didn't know how to test WSs which means there's a lot of shit to sift through and you can't always trust the results.
I'm 100% sure at least one of their WS mods/fTPs is wrong (Pyrrhic Kleos), but other mods of theirs match up with things I've observed better than ours. For instance, a while ago I wasted a bit of time trying to find a hidden effect on Twilight Helm because Primal Rend was getting extra damage with it compared to 10 INT/CHR. If Primal Rend is also a DEX mod, it would make sense. It would also make me pretty incredibly lucky for not switching in/out DEX gear when doing my killer tests. The magic WSs, at least, should take about 30 seconds in Abyssea to test and verify.