It's hard to compare this without the powertap hub currently working, but I find that my HR on the bike on the road runs higher than when I am on the turbo. When I am at 160bpm on the turbo I feel I am working very hard, my legs are burning etc. The same thing on the road, my legs are fine, and my breathing is much slower than on the turbo.
Does anyone else notice this. Is it an adrenaline thing, perhaps?