From the point of view of the 60KWh customers, they're still getting the same product for the same price that they agreed - so if the product was worth it to them at that price before, it should still be, regardless if some other customers are getting a greater surplus. Other people getting a good deal doesn't affect your utility.
This is so even if they were just shipping the 60KWh model as-is to those lucky 40KWh customers.
Yes, except the 40 kWh customers are essentially getting a rebate ("20 extra kWh" for free). The 60 kWh-ers may reasonably ask why they don't should not deserve a similar rebate (eg., free upgrade to 80 kWh), especially considering they paid more for the product than the 40 kWh-ers.
Note that I'm agreeing that it's nothing to get angry about (and we don't know whether this is something that can be "unlocked"), but I disagree that it's some kind quirky human, irrational response. On the contrary, it's completely rational; it's rational to recognize when something is distributed inequally in a way that favours some people out of sheer luck.
This is so even if they were just shipping the 60KWh model as-is to those lucky 40KWh customers.