cycling heart rate

yes, It was much easier to ride at what I thought was an aerobic heart rate (155) on the trainer. I even did a few tempo rides on the trainer, keeping my heart rate at 170 or so. Now that I am biking outside, my hr is about 20 beats lower (135). When I climb hills it increases up into the 150s, but otherwise, I have to be pushing pretty hard, at a pace that isnt very comfortable, to get to 155. I thought that my muscular endurance was good because of my indoor riding...now im not so sure. Is this a fairly common experience from riding indoor to outdoor?

My running has been about the same when riding indoor and outdoor. Im training more now, but I am reaching a higher volume portion of the season. But I dont feel overly tired when I ride. I actually feel really good, its just that I feel like im not getting an aerobic workout. And I dont necessarily think I am going to easy. Generally riding at around 20mph. Even last summer when I rode outside, my heart rate would be in the 150s most of the time. Maybe it is due to a difference in position while riding this indoor/outdoor year versus last summer (have only been riding on my TT bike instead of the road bike) so my heart is more level with the rest of my body. It seems that most inexperienced bikers have lower aerobic hr initially until they improve their muscular endurance. But if I was riding in the 150s for bpm last year, I feel I should be able to continue riding at that aerobic level. Or is this expected once you become more toned for biking?