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?