Fully agree - not the most polite reply, n3mo. Please take a look at our Community Standards if you haven't already (which you agreed to on signing up). We don't ask a lot from our members, but we're all adults trying to help each other out, so there's no good reason for anyone to be impolite.
Erik - To (try and) answer your question, it doesn't seem usual to me - but it would be helpful to know if you changed the details immediately - or did you run one quote and then change the details and refresh the quote an hour or more later?
n3mo is right in one sense - premiums do work differently - and insurers are constantly calculating and re-calculating their approach/attitude to risk - so quotes can and do change depending on the details you enter and amend (obviously).
However, it's still a little surprising that the quote marginally went up when you entered a lower mileage. If this quote was generated through our site, please feel free to drop me a line and I'll be happy to ask the boffins to look into it for you.
I hope that helps,