I am pretty sure many math grad students are not that great at mental arithmetic, which is the topic here. We are obviously not talking about the kind of advanced maths grad students to.

It is an interesting topic. I remember seeing an article about teaching maths to shopkeepers kids in third world countries. By western standards, they are quasi-illiterate. They are however remarkably good at tasks like giving change. When asked about their reasoning, it involves exchanging bills and goods rather than abstract numbers, and IIRC, they do better than western schoolkids of the same age.