Ah, thanks Pleiades.

So they do all have basically the same engine after all, and it's rated at 35.0 kW. The A2 limit is also 35.0 kW.

Apart from the XT660X being left off the list of A2-friendly bikes, it appears my confusion came from the horsepower conversion.

Apparently 48 PS is not the same as 48 bhp - I always thought it was near enough to assume 1-to-1 conversion. But after a bit of hunting for conversion factors I found:

Using the conversion 1 horsepower = 0.7457 kilowatts = 1.0139 PS,
then 35.0 kW converts to 46.9 bhp or 47.6 PS.

Cheers!