Don't get me wrong, I'd also be frustrated. However, if you view the grades as cutoffs rather than rounded percentages, I think the professor's view makes sense

Don't get me wrong, I'd also be frustrated. However, if you view the grades as cutoffs rather than rounded percentages, I think the professor's view makes sense. The cutoff for an A- is .90 Therefore, if the score was 0.8994, then you did not make the cutoff, and your percentage grade of 89% would be more appropriate.