RPE and HR on road vs trainer

I've noticed that it seems harder to get my heart rate up to a certain level and keep it there on my trainer than when I am on the road. For instance, to get my HR to, say, 85% of max on my trainer I have to work fairly hard. But when out on the road I can get there usually quite easily. And it is sometimes not possible to get up to 92-94% max on my trainer for really hard efforts although I can ride at this level on the road. This is with a CycleOps fluid trainer but I've seen the same thing on my former magnetic trainer.

Any idea why my RPE is higher on a trainer? Is it due to the resistance settings or maybe relatively poorer cooling effect? Has anyone else noticed this with their training, and does it also occur on rollers or if trying to reach a certain wattage?