I've noticed that sometimes you can get more Skill proficiencies than you're supposed to have. I think the problem is the Ranger's "Urban Tracker" specialization.
It appears that the Thieves' Tools proficiency is implemented by granting Sleight Of Hand proficiency. However, if you select Urban Tracker and then de-select it, the Sleight of Hand remains permanently on the character sheet. Not sure if there's other ways of getting bonus proficiencies, but this appears to be a bug.