So, almost exactly a year ago I went to an opticians for an eye exam. With the pandemic it had been almost 6 years since my last test and although my eyes felt stable, I thought it would be a good idea to get them looked at.

I booked at a fairly highly regarded small national chain in the UK. I was shocked to be told that one eye had gone from -3 to -4 and the other from -3 to -3.50 and that in the meantime my slight astigmatism in one eye (-0.25) had become significantly worse. At the time I was surpirsed for two reasons. First was that I had been previously told by an optician that my eyesight would change very little in my 30s and secondly I felt there had been no noticable change in my eyesight and my glasses still worked perfect well - I could read things far away comfortably for example.

Cut to yesterday and I went to a more expensive independent optician (literally about 10 doors down from the chain) for another exam, partly because I never trusted the results of the last one. First thing the optician says is "wow, that's a big change if it's correct". However, within 5 minuets of that she had already said the presecription was "wrong" and that I should just "throw it in the bin". After doing a more thorough test, I was given a new presecription with both eyes only changing only -0.25 to -3.25, and with no change to the astigmatsim.

My question is how did the other place (assuming it is the other place and not the independent I went to yesterday) get it so wrong? If I had just taken their word for it and got new glasses I would have been walking around with a presecription that was far too stong for me. The more I think about it, the more it feels like they really must have messed up somehow?

If anyone with more knowledge could let me know how this might have happened or if it is something that just simply happens every now and then I'd really like to know! Thanks a lot if advance.

TL;DR: Have been given two very different prescriptions in the last year. My recent optician told me the previous one was "wrong". How does this happen?