4.5 miles/kWh is hugely optimistic, and a 30kWh reserve is highly unrealistic. Tesla's range numbers are based on roughly 3.3 miles/kWh (which is reasonably realistic) and the floor is maybe 5kWh.
I can't explain the discrepancy, but it's definitely not just the floor. Either they've made some sort of efficiency improvement, or the difference between the two battery packs is more than 10kWh and they've just rounded off the figures.
If the floor is X kWh, then we can solve for X based on these two ranges assuming the kWh / mile consumed is the same ratio, r.
Solving these two equations gives me X = 30 kWh as a fixed reserve in the battery packs assuming r = 4.5 mi / kWh.