I recently posted about this
here.
A change of 18% is nothing after 1000 hours. You only have 68% confidence that your hourly rate is even accurate to within 32% assuming your standard deviation is 10 times your hourly rate, so it can easily be off by even more than 32%. This is assuming there was no change in your competition or in the quality of your play. If you want it to be accurate to within 18%, you'd have to play 3086 hours to have a 68% confidence of being that accurate. If you want a 90% confidence, then you'd have to play 7901 hours.