@jenna_miltenoff -
Well you have an interesting problem. Thanks for a great picture that explains and shows everything. My first guess was going to be the grading scheme, but your picture shows the one being used. As you stated there is no B- in it, so why is the student seeing B- especially when the instructor sees the C. You also clarified that there were no hidden grades.
Based on everything you have stated, it sounds like you need to send the information off to Canvas support. contact your Canvas admins on how to do that - or see if they can help any as well.
I imagine you have already done this, but I would double check the grading scheme for the course. And just another thought - are there any assignments that have a grading scheme assigned to them? if so, what are those grading schemes and is that a possible source of the issue?
If possible, can you create a dummy student and give them the same grades to see what their score looks like? Or try it with the test student?
Ron