How much was the difference?
If it was just a few pounds it is likely to be a rate change, like how exchange rates change regularly, if it is a large amount it seems a little unusual, maybe double check you have entered the same details, perhaps you added a named driver on the first quote and not on the second one?
Also have a look at Marmalade insurance they do blackbox insurance for young driver I don't believe they are on comparison sites, they may offer a cheaper quote
http://www.wearemarmalade.co.uk/?guid=31FE27A6E21C4F45980481B479E8448BAlso try other comparison sites, I like the one where you get a free meerkat! Much better than a free robot toy or annoying opera singer. Some comparison sites charge more commission to the insurance than other, therefore meaning there can be a price difference. I have even found that if you find the insurance company on a comparison site and then get a quote directly with the insurer the quote can be cheaper (possibly as they don't have to pay commission to the comparison site)