Either way, the capacity lowers as time goes on. Your 60 kWh battery might become 50 in a few years. The extra 15 is supposedly a buffer against that.

It’s the same thing with spinning hard drives and SSDs. The reported capacity is almost always lower than the actual capacity. That allows dead blocks to be “replaced” without the user even knowing (“remapped sectors”). Could you imagine the PR disaster if your SSD lost visible capacity as time went on?

You do have a point about the weight. The cost of shipping an SSD with 0% overprovisioning and one with 25% is negligible; the ICs weigh just a few grams. However, a car that has to haul an extra 15 kWh of unusable capacity does use extra energy. I’d be curious how much is needed to haul those extra cells.

Sure, but using 60kwh of a 75kwh battery lasts longer than a 60kwh battery would. Not to mention that Tesla's R&D to make a 60 kwh battery, an extra asmembly line (or one flexible enough to produce both), extra inventory, testing, maintenance manuals, etc all has costs as well.