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"I can't think of a good reason either". The frame builder said "It must be 70mm" so either he doesn't remember or it got modified for some reason. 1mm spacers are not hard to come by so it's not a big problem. I used mavic elypse wheels cause they have a wider chainline. Hallowtec cranks also have a wider chainline. It can be a bit difficult to build this with classic track components, but it's quite easy using modern standard parts.
Yes, this is custom frame made this way.