According to my mental model of the world, if I'm trying to find people who will pass their interviews at IBM, then the more my qualifying interview looks like IBM's interview, the better I'm doing.
I find it very plausible that your experience ("acceptance rate doesn't seem to change no matter how I personally vet the candidates") is more realistic than my armchair model, but I suspect the model is intuitive to a lot of people and will go a long way toward explaining "why are you asking questions about relational databases?".
