I have had interviews where the questions asked were not in my forte. I studied math, but worked as programmer for most of my life. At my university, I remember seeing certain classes taught for different audiences. Numerical analysis taught for CS majors was completely different than that taught for math majors. CS people devoted a lot of time to computational complexity formulations for various sort algorithms and approximations to those. Math numerical analysis chose different problems to study. Even year to year, the math problems studied varied based on the professor.
Take differential equations. DE for engineers emphasized recognizing and memorizing certain route formulations for solving "standard classical problems." DE for mathematicians involved heavy theorem proving and esoteric questions like "existence solutions". Engineers would laugh, saying why the hell would we need to know that? Well, the answer is simple. Most of the easy differential equations that "you can solve" can be solved by looking it up in a book or these days, via Google. However, in the real world, you will most likely encounter fresh ill-posed "one of a kind" DEs. Another example happened when an engineer asked me help with solving sets of DE when the Space Shuttle's robotic arm hit an object. The way that we formulated the solution then was completely wrong. To deal with the impulse event, we switched from a high order RK method so a lower order approximation after impact.
When I took my GREs in math, I bombed my DEs part. Well, the reason is that all of the questions asked focused on the Advanced Nonsense taught to EEs and not on stuff which I was taught.
Years later, I had the exact same problem during an interview with Microsoft. I didn't do well because the sort of questions asked came from the CS school rather than the math school. I don't mind that, but I resented him because he just thought that the interviewer dismissed me as an idiot or fraud because I did not know the answers to his questions. Had he said, "Well, you just MIGHT know your shit, but it is not the shit that we do here, and I do not have a way of evaluating your skill set," I would have accepted that compromise.