I first encountered humans who couldn't understand basic arithmetic at university

Thinking about this a bit longer, I think mathematical logic is a good example that shows that their problem is unlikely to be that they are fundamentally unable to understand basic arithmetic. Logic is a "system of inference rules for mechanically discovering new true statements using known true statements." Here the emphasis is on mechanical. Is there some sort of understanding that transcends the knowledge of logical symbols and their truth values? Is arithmetic particularly more demanding in this respect?