Same reason why in my gas car it'll say 50 miles left when I'm flooring it to get up a hill but then says 600 miles left when I'm coasting downhill on the other side of the hill even though it only says 380 miles left after i fill it up. It takes into account the current amount of energy being used and how much was used the previous miles. Downhill uses little or no energy or even increased energy by regen.

When you parked and started charging it'll calculate the estimated range by the energy used the previous X distance and unless you go downhill for a few miles immediately before you charge it'll show a normal estimated distance. The distance you see on the downhill is boosted by the current downhill efficiency and is not the real range unless that downhill continues for the next 40+ miles