most of what I have read points to what mich said. the AR just becomes less sensitive over time while on cycle. Although, arachidonic acid is supposed to increase the sensitivity of the AR. From personal experience with running X factor from MN, I could definitely tell the difference.
I ran a methylsten product and at the 3rd week mark I hit a plateau in my weight, I just was not getting any bigger, I threw in some X factor and started gaining significantly again.