In this IBT, pickpocketed items do not carry over from one stage to the next. So that potion from stage 1 wore off. On this stage, there is a class rule that gives wizards a -1 modifier to their rolls on this stage. The correct resolution of this roll was: 8 - 1 (wizard) = 7. So you performed better than you originally thought.

In future stages, please make sure that you account for all stage modifiers and that you only apply item modifiers from items that you acquired or activated on the current stage.