I really don't see how any random sampling mechanism (unless you're literally stratifying based on the last digit of the odometer) would cause these sorts of results, please explain further.

These are customers they already know I presume. So they have an inkling of the expected mileage (at least: we price on expected mileage). So you repeatedly sample dropping some samples to get enough filling in all mileage baskets. It’s a stratified sample and random.

Say you have 10k customers with 1k women. You want to take a sample And want enough power to get answers on women. In that case you do stratified sampling.