There have been numerous studies about how your body can 'adjust' (that's not the word I really want, but it'll do for now) to certain activities, become more efficient and therefore get less of a benefit from those activities. This is why weight lifters have to change up their routines every 16 weeks or so and why there is such a huge trend towards periodization in so many sports.

I don't know that there are any cycling specific studies (there may be), but I would bet that you could find lots of examples for other sports if you searched under the words 'fitness plateau' and/or 'training plateau'.