Could be an issue of resistance too. If the resistance is set really light and isn't progressive as you speed up (like a cheapie mag trainer) you can go pretty fast. Averaging 30mph on a Fluid2 or some other progressive fluid trainer that's properly set up will smoke you. My fiancee uses my Fluid2 a lot, and her indoor speeds are very similar to outdoors in the flats. Me? My speed sensor is in front.
I totally agree with the HR issue on a trainer. I think there's got to be an environmental reason for it, i.e. you're not using any energy or brain power watching the road, steering, braking, etc., not to mention the inherent wind resistance on an outdoor ride. I don't think it's really an issue of aerobic capacity, but that's a total guess.
DrPete