I just noticed something weird with the 305 when I'm doing intervals on my trainer.

During the 4 minute "on" phases it shows an ascent of 6m and a descent of 6m yet during the 2 minute "off" phases the ascent/descent reads 0. Of course on the trainer in my apartment I'm at the same hight all the time.

The only explanation I can think of is during the time I'm spinning I'm riding in position so my mouth is nearer to the stem/305 and when I rest I sit up straight in the saddle. Could it be that my exhale/inhale during the "on" phase is changing the air pressure around the 305 enough to fool the altimeter?

Anyone else notice this/have a theory?