Is there a reason why one's calfs would ache on a Turbo or Spinner, but not when riding on the road? I'm not on my own on this one as at least two other people in my Spinning class suffer the same:  when Spinning or on the Turbo Trainer, I can feel pressure on my calf muscles after a bit of time (usually less than 45 minutes).  But I can ride on the road for up to 3 hours without feeling any pressure.

I think it's something to do with leg angles:  on the road, the bike changes angle slightly to best match the angle of the leg, but on a spinner or Turbo, it's fixed and so can't move.

Are there any proven reasons why calfs would ache on a static bike but not on the road?
There are likely a few different reasons, but my primary suspicion would be fit. Presumably your road bike is set up to fit your body geometry and angles, where the spin bike is not. Small differences in how a bike is set up can have a big impact on how your body feels while riding.

My n=1 example, when doing higher intensity efforts on the road, I would get high calf cramps after about an hour. When I had a Retul fit done for my time trial bike, the fitter told me that because my feet were somewhat duckfooted, I needed to move to speedplay pedals with longer spindles to allow my heels to drift inward more than my old pedals. Solved my calf cramp problems.