Interesting. Given that bosses have only 10% defense though, wouldn't that indicate that special attacks should almost never miss? Combat logs don't seem to support that. I assumed it was only a 5% boost since my Spinning Strike seems to miss 4% of the time on average.
101% Special Accuracy. Between needing 100% to guarantee a hit, and then 10% defense chance to deal with, you should expect to see ~9% missed Assassinates/Thrashes, though the low usage count means the sample size is tiny and variance in miss rate will be huge.
Once your Accuracy passes 100%, you can't outright miss a mob, however they can still defend (via parry/dodge), and the reason DPS classes dependent on white damage go to ~100% base Accuracy is to push their Special Accuracy to 110% and negate that defense chance entirely.
The 10% defense chance is in addition to any innate possibility of missing via being below 100% Accuracy.
Edit: This is easiest to see/understand looking at a dual wielding class. Offhand accuracy never passes 100%, so you *will* see missed hits that are not dodged or parried because they never landed in the first place.