Are we sure this is comparing apples-to-apples here? It's always been slightly curious that there's only one EPA number for the 85kWh - you might expect the Performance and standard models to give different results (due to the 21" tyres at least - and it would be surprising if the powertrain has no effect at all).
Maybe the '85' number is for a performance with 21" wheels while the '60' is on the 19".
Another variable is the reserve capacity. It's reasonable to expect the top-end reserve (difference between standard charge/range charge/theoretical max charge) will scale in proportion to the pack size since the limits correspond to cell voltages, and so that wouldn't affect the relative range. On the other hand, the bottom-end reserve (2 months anti-bricking allowance after reaching 'empty') should relate to the vampire load and so should be a fixed number of kWh rather than a percentage of the pack [if I've analysed it correctly, that should actually give worse range for the 60kWh rather than better, but at least it's an example of a factor that doesn't scale - perhaps there are others].
There's also the question of whether the reduced performance of the 60kWh means that it ends up doing less work during the EPA test. I can't find enough detail about the test cycles to say if this is an issue or not.