Don't over think it, it's an auction.
For the reported week, there were 21 cars that were on the block and for each condition level, the mileage reported is the average of all of the cars that fit into that condition level.
Typically low mileage cars are cars that come from the manufacturer which have been used for events, promos, as executive or internal use cars. Other sources are dealers that have cars in which the deal went bad after the car was delivered and the transaction had to be unwound (usually credit problems of the buyer). Those cars, while essentially new, have to be sold as used because they have been titled. Other cars that fit this lower mileage profile are lemon-law or manufacturer buybacks which are sold with a marked title. Higher mileage cars are typically lease returns. In this batch there are 4 lease returns that had an average mileage of 15.5K.