now I can see we are talking about two completely different things because you cannot explain yourself clearly.
WTs have a bonus that reduce the damage done by enemy provinces, such as less troops killed in a NS against you.
Of course when you sent higher amounts of thieves on an op you will lose more, that is obvious to everyone. I said the damage against you is reduced by your WT bonus, regardless of what your defending tpa is, even if it is 0 tpa.
WTs DO NOT modify how many thieves the enemy loses on an op, it only modifies the chances of him failing.