The Spinergy Rev X series came in the following variants:
26" (MTB)
650C (Triathlon)
700C (Road)
Indeed axle widh differs but also note that rim diameter is not the same for either variant. MTB tires do not fit on the 650C rims.
It is possible to fit 650C rims on a MTB. Only the brakes to have to bre adjusted slightly higher on the cantilevers/brake bosses. I use the 650C variant on my bike for road trips.