^^
Interesting that the range for the Ego+ is no different.
As for the source question, the primary purpose of publishing performance/economy/range specs has always been to be able to compare between vehicle models rather than get accurate numbers, which after all depend greatly on the specific conditions. That means that a standardized, detailed, and published test procedure is a requirement. A vendor can always publish additional numbers if they think they reflect "real-world" results better.
This is all the more relevant for motorcycles, where the form factor & weight of the rider, luggage, accessories etc. affect things a lot more than they would a car.
Zero does say in the specs for their "combined" range that they measure it according to SAE J2982, but don't give full details for the other cases (the SAE doc isn't available for free AFAIK).
IIRC, the SAE standard is done on a dyno, so wouldn't necessarily very realistic w.r.t. aerodynamics, although they use some computed formulas to compensate.
I don't know if there's a comparable EU standard that Energica could use, but I suspect not -- Zero quotes the SAE standard on its EU website as well, and if there were an EU standard, they'dmost like use it.