Here let me dig into my image macros to find the appropriate one for this thread:
So the reality of practically all Li-Ion battery systems and chargers is the fact that you don’t use all the given capacity. Li-Ion does not like to be fully discharged repeatedly, doing so will seriously shorten the life/capacity of the battery. Now that also doesn’t mean you want to top it off to full 100% capacity at every charge either. So in a 75kWh pack you probably have around an actual 50kWh of capacity to use. This doesn’t just go for Tesla, this is all electric cars, cell phones, laptops, practically any Li-ion powered device. A cell life cycle is based on chemistry that is rated off the actual full capacity of the cell - which means a single cell that holds 12W, but was charged three times each with 4W would have been through one cycle. In reality it is much better to look at range vs. pack capacity than it is simply just 60 vs. 75.
Like some people have mentioned upthread the 75kWh pack rated at “60” would last longer simply because you are using less capacity. So Tesla just unlocked that extra 15kWh…in reality they could have relaxed the remaining charge percentage lower and given you more range temporarily on every model - obviously at a minor hit to future pack capacity (using a SuperCharger repeatedly probably has the same affect on capacity as well.)
As far as them putting a 75kWh pack in a model sold as 60… No one looks at power tools do they? 18 to 24 months ago a you were seeing the extended run pack capacity at 4Ah, while base unity were typically 2Ah. Now you see base units at 3Ah, extended are at 6Ah (a few larger packs go to 9Ah). In reality this means the base single cell capacity increased from 2Ah to 3Ah, or a 50% capacity increase. I’m sure Tesla and every other EV manufacture has taken advantage of this, which is why that new 75kWh pack probably cost Tesla the same as the original 60 (it wouldn’t surprise me if it was smaller and weighted the same or less). From Tesla’s operational stand point it makes no sense to continue to have two batteries when one can be utilized for both applications and the consumer end result is no different.