Correct, I'm not comparing valuations made on different days for the mileage analysis, they are made on the same day.
Basically what I did was refresh the quote, got £7160, but then figured I ought to update the mileage as well. So I increased the mileage and my valuation increased to £7205. To make sure, in case they had updated their stats inbetween my valuations, I tried dropping the mileage back to 65000 and the valuation dropped back to £7160
The only thing I can think is maybe they have some algorithm that prefers more 'precise' mileages, i.e. people that put in lazy values rounded to the nearest 5k get lower quotes on the basis they are lazy people that will just accept lowball valuations. But experimenting it wasn't that cut and dried either, it's not like 62250 gave me a higher valuation than 60000.