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.
It would indicate that melee special attacks get avoided by the bosses 10% (or 9% if you have +1% from companion buff) of the time. You would need 110% melee special attack accuracy for the special attacks to hit 100% of the time. Note that 110% melee special attack accuracy is reached at 100% melee normal attack accuracy, so you still need only 100% melee accuracy for the special attacks to not get avoided. Normal attacks (Saber Strike only) can still be avoided at that point.
If special attack accuracy was only +5% (as in 95% base accuracy) then you should expect them to miss or get avoided 15% of the time against a target with 10% defense chance, but that does not seem to be the case.
Force and tech attacks of course do always hit even without any accuracy rating and they do not benefit at all from accuracy over 100% (which is their base accuracy) since pve enemies, not even ops bosses, do not have any avoidance (resist chance) against them, (unless you are low level character trying to kill mobs over 5 levels higher than you)
Not that I think that the ingame tooltips are always completely reliable, but just look at your character sheet and move your mouse over your melee accuracy and read the on-mouse-over popup tooltip.
It should look like this:
So melee normal attacks have 90% base accuracy and melee special attacks have 100% base accuracy.
In this case I think the tooltip is correct.
As for the notion that operation bosses have 10% defense chance, I'm not so sure about that.