That's because the bump didn't give you anything besides software change that estimates the range further. You didn't actually gain more range.
The LR RWD was rated at 310, same as AWD, but EPA ratings for it is actually 331 miles. So they simply updated the estimated range.
What we're seeing now is different, it's actually claiming more range than EPA states for AWD. So I expect this bump up to have real range increase.
I see this thrown around a lot but the evidence does not support it at all. The change you're implying (a change to the Wh/m used on internal calculations) would have also increased the miles per hour charge rate displayed on our home chargers (at least if charging at 40 amp). This didn't occur. 40A@240V before and after the change are still 38 miles/hour. The change that you assert occurred would have pushed it up to 39.
The other (and more likely) theory is that the LR RWD originally had a software lock on part of the battery capacity to keep the range more in line with the AWD, and that was removed.