There are other changes that go on that you may or may not notice.
My FTP increased after my first year of riding but has essentially stayed within a range of 10-15 watts ever since.
My ability to ride now for longer durations has changed significantly. I am able to comfortably ride for 3-4 hours at a pace I couldnt even maintain a few years ago for an hour. My stamina has changed dramatically. My resting heart rate dropped dramatically as well. It all took time and didnt happen after a short block of work.
Before you took the test did you think you were improving? Is it just the result that is causing the disappointment. Dont focus on how you perform on two specific days. Work through it and see how it goes.