First, those mileage figures are what the tire manufacturer provides as warranty - and the warranty stipulates regular rotations, which obviously didn't happen to the set on there now - meaning the rear tires can go a lot further and that improves the average.
Second, tire wear is highly dependent on the driving situation. Most tire wear occurs in the cornering mode, so driving in the city with all those turns wears tires out much more rapidly than country driving, where straight ahead driving is practically free!
So take those mileage warranty values with a grain of salt. They are just what the tire manufacturer feels comfortable doing a warranty on - and some manufacturers can get pretty aggressive with their mileage warranties. (Michelin tends to be conservative.)
Then there is the issue of OE tires - the ones that come on new cars from the assembly plant. Those tires were designed by the vehicle manufacturer when it comes to wear, traction, and fuel economy - and since vehicle manufacturers don't care about tire wear, but do care about fuel economy, they specify tires that don't wear well, and may have traction issues in order to get tires with good rolling resistance. So the tire manufacturer doesn't provide a mileage warranty on OE tires (with a couple of exceptions.)
Please note: It is common for tire manufacturers to advertise a mileage warranty for OE tires that they sell through a tire dealer, because tire dealers want that, but the tires that actually come on the new car don't have that warranty. This can cause confusion for the consumer. This is especially true for tire lines where most of the sizes are NOT OE, so the tire manufacturer will just have to eat the cost of mileage warranty claims for those OE sizes sold through a tire dealer - sort of a cost of doing business.
Another consideration is that different tires front to rear can cause handling issues in emergency maneuvers - and sometimes those handling issues don't show up until you need them to behave predictably (Let's face it: Most people don't do emergency handling maneuvers unless they have to, so they never know!) - and then it is too late!! And the more different the tires are, the greater the risk.
The best situation is to have 4 tires of the same make, model and state of wear. Next best is the same make and model.