Routers are complex. Yes, there are standards, but standards are not always interpreted or implemented exactly the same. And bugs, of course. Routers are mostly software, and like any software they often have bugs.
Most manufacturers test very carefully against Windows and as much of the popular windows hardware as possible. They also test against Apple hardware, but given how many more Windows computers there are in the world it would be surprising if they didn't spend most of their time and budget there.
Apple, on the other hand, is going to test its routers with its own hardware first and worry about Windows second. If Airport routers didn't work better with Apple products it would a major failure on their part.
In the end, most routers work with most computers most of the time; but when they don't it can be almost impossible to figure out why; because routers are complex.