"Unspecified chassis alterations" and more efficient tires are two reasons behind the 8% range improvement.
The WLTP rating is now 484 km (301 miles) - about 35 km or almost 8% more than previously in the European version produced in South Korea, which was rated at 449 km (279 miles).
The press release explained that Kona Electric from Czechia is equipped with new, very efficient Michelin Primacy 4 tires, but that would not explain how the car gained 8% (unless previous tires were borrowed from a tractor).
According to the latest media reports, Hyundai conducted "a handful of underbody revisions to help reduce mechanical drag and friction with the road surface".
"Unspecified chassis alterations and new Michelin low rolling resistance tyres have been brought in, to increase the officially quoted WLTP range from 279 miles to 301 miles. The battery pack remains unchanged, as does the car’s 201bhp output and charging rates."
That sounds more reasonable and proves that EVs are far from reaching full potential if you can gain 8% just on the mechanic/aerodynamic side.
We must remember that the Kona Electric, although a great EV, is not even a bespoke all-electric car (there is also a conventional and hybrid version) so a lot can be improved.