Soldiers can ride horses, mercs can't. 20%+1 is what I've heard for SoM/SoT to have a 5%+- accuracy. Tacticians have 100% accuracy in war time even if you send 1 thief (assuming you do get the intel though).

Also to elaborate, the 20% is of your own maximum amount of thieves. Your TPA is taken into account with the probability of your op succeeding, has nothing to do with accuracy.