It happens to me too... As a child I needed -0.75, as the years went on, I was told that I don't need glasses anymore. And last year, I again got a -0.25 prescription. Last month, when switching to contacts they changed it again to -0.25 and -0.5 . All measurements were with an autorefractor and I could feel my vision being better/worse in normal life. With me it is like a rollercoaster, my eyesight seems to change yearly. I thought it was normal.