Did you use guidance spell?
It gives a 1d4 bonus to all skills and it is rolled twice, first when it shows the skill check and second when it rolls the skill check.
This way success or fail can be wrong by up to 3 points.
Wait, what? you are saying those two 1d4 rolls are different? That actually explains a lot of weirdness with dice I experienced. I.e. succeeding on 11 roll after rolling just 10.