The answer is chunking. Concepts and categories can be organized in many different ways. How you organize them affects the speed and ease with which you can think about them.

Say I'm trying to keep a phone number in short term memory. If the number is 546-9135, then it's going to be difficult. But if the number is 123-4567, it's going to be easy. Why the difference? The second number can be chunked, the first cannot. The first number takes seven slots or so. The second can be chunked into one slot: Numbers from one to seven.

How about 123-5813? No one but a mathematician would be able to use less than three slots for this one. (numbers from one to three/fifty-eight/thirteen). But if you can recognize that this is just the Fibbonaci sequence, it fits into one. So, expertise and experience help us to chunk better.

This is one reason why math problems get easier with practice.