It may be that chess skill (the way I see it anyway) comes in two parts. The knowledge and the application (theory and practice if you will). You can learn a lot from books, but then you have to internalize it in a way that you're able to apply it, and then... you actually have to apply it to your games, which can be quite a process.
In the meantime you've likely acquired a good amount of knowledge and lessons from playing... but you can only practice, or try out so many new ideas at once. This block of new ideas transferring from theory to practice within an individual over the course of many games eventually reaches a point where it crosses over. Not only have you put into practice 1 idea, but the whole batch as well, and so your results (and your rating) jump at once.
Back to the learning and incubation period, now we have a plateau where you may even get slightly worse as you gather information and try to find the correct practical implementation for the next batch.
This is one way you can think of it anyway.
why is it that improvement always seems to come in 100-200 rating points at time with long periods of leveling off?