How often does power decrease over a period of a year?

My prescription came out last year to -5.5 and -4.5. As a side note, this prescription was provided by an optometrist who was in a REALLY bad mood, was running behind already and reminded me of it about 10x during the appointment, and then the whole office's computer system shut down at the end of the exam, which just about sent her over the edge. She gave me my prescription based off of 'what she thought it was, thinks she remembers' and sent me on my way. Yes, I should have said something and she was really, really unprofessional about it all the entire time. Anyway…my prescription this year is coming out to -5.25 and -4.25, so not as strong. This was done at a different optometrist who has great reviews and seemed more than competent. Is it normal for one's prescription to go down? I've read about damaging your eyesight if your prescription isn't spot on, so am getting a little paranoid.

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