@jfurby
The first thing I would do is to double check the point values on the assignments. If one of them is worth 0 points instead of 10, then a student who got 10 points on both would have 10 points + 10 points = 20 points, but Canvas would think it's only out of 10 points and give 20 / 10 = 200%.
The 15% is irrelevant if both grades are in that one assignment group and there are no other assignment groups with anything in them. Canvas doesn't include assignment groups that have no grades in them and, until the percentages reach 100%, it scales. If you have 15/20 points = 75% in an assignment group worth 15% of the grade, it's still a 75% for the course. If you had 75% in an assignment group worth 15% of the grade and 20% in an assignment group worth 30% of the grade, then your student's grade would be (0.15 * 75% + 0.30 * 20%) / (0.15 + 0.30) = 17.25% / 0.45 = 38.33%
If that doesn't resolve the issue, can you provide screenshots (without any identifying information) of the 1) gradebook with the individual grades and how much they're worth, the total column of the gradebook showing the 200%, 2) the assignment group weights, and 3) and rules from dropping you have in place.
Sorry to ask for so much information, but we're a global community of Canvas users and don't have access to your course to look for ourselves. You may also be able to get help from your local Canvas administrator who would have access to this.
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