Assign Consistent Letter Value To Grade Percentage

Newbie here. I have a very frustrating problem. I am using excel for my gradebook at school. I've tried several different ways to assign a letter grade to an number average. It works fine each different way I do it, VLOOKUP, IF, etc. But, I have about 5 grades out of 100 which give the wrong letter grade. For instance, my scale says that a grade of 85 should be lowest limit of a "B", but I get a "C" returned in the cell instead. Like I said, it only happens on a few grades. The biggest majority work fine. I can't figure out why. Any ideas? Attached is one of the "problems" with a student's grades. Note the Final Avg with a grade of "C", it should be "B".