I've noticed this too and have been trying to keep an eye on why. I have noticed that when I have quite a bit of + to the roll (dex, sleight, spell bonuses.) and it's a 10 or below, it doesn't even light up the thieves kit as being used. When it's a higher requirement of 15+ OR I don't use the spell bonus, or I use my main character who has the dex bonus but not the sleight of hand..it doesn't use it on lower requirements. My guess based off this was that if I blew the requirements away (5 or 10 maybe over the requirement?) it wouldn't even bother using one. It's all conjecture though, but that's the trend I've seen since I started paying attention.