There are a few reasons that this could be happening.

It could be because an Assignment has a max point # entered but the points have exceed that max #.  For example, a Quiz has a Max of 5 points. If the  points earned are over 5 such as you give the Student a 10, this would then put the Student over a 100 for a grade because it is saying they earned 10 out of 5 possible points.

This could also be because of Extra Credit being given, for how Extra Credit is calculated see Calculating Extra Credit.