Car insurance quotes do seem almost random.
My renewal in December was £68/month, ~£800 for the year.
This was with Direct Line (I had a fairly good idea I could have got my insurance cheaper had I shopped around, but for whatever reason I never got round to it).
Come last month, wanted to add my wife as a second driver.
Direct Line wanted an additional £23 per month, which would have taken my premiums up to £91/month.
I complained a bit and asked if they could beat it, but the sales person on the phone basically said "computer says no".
So I cancelled (£26 charge), and found an identical policy with Aviva for £51 per month (although this is with 2 months 'free') for both of us. Plus I should get £65 back through Topcashback.