altitude, higher altitude, and heart rate

Hi,

A friend asked me a question, and I was confused by it so I thought
I'd throw it out to the group for your input. I know that this is a
triathlon group and my letter pertains to skiing, but I thought I'd
tap your heart rate knowledge.

He and his wife live in Colorado Springs (altitude 7k? or so). They
both skate ski with the main race being the American Birkebeiner each
year. This year they started watching their heart rates more closely
with the aid of a HRM while they've started their early season
training.

The on-snow training that they do is in Breckenridge (altitude 10k),
however, they roller ski both in Colorado Springs and Breckenridge
when there is no snow.

The have been finding that, even though their roller skiing perceived
exertion at the higher altitude in Breckenridge is higher, their
threshold heart rate is lower. Doesn't it stand to reason that their
heart rates should go up at altitude instead of down at the same
perceived exertion due to the lower partial pressure of oxygen? Or do
the muscles fatigue out due to the lower oxygen and the heart rate
never has a chance to get up to the higher level?

Thoughts? Has anyone else experienced something like this?

Brian