Ye, took me a while to get my head around it, but it seems to be a legitimate thing. Welcome to google '3 phase no neutral floating earth system'. It's explained pretty well there.
My initial theory was, a line was drawing more current than the others, creating the imblanace, thus leading to the 70v on earth, but no luck chasing that, with the megger on 240v testing around for shorts to earth, found diddly'

There's no stand alone transformer as far as I know. All the motion and spindle controllers are directly from the 3ph 240