Resistance or tolerance to gear is a real thing, and that's the only way to explain why, while AR upregulates from using steroids, we are getting less and less results after 5-6th week of the cycle, and why, if , after 8 weeks we change the used steroids for others it will boost some more growth.
The whole process take place in brain and liver, same as with any other medication. Brain monitor the whole body as one system to adjust to changes and either eliminate them or addapt to them, liver's enzymes become more and more effecient in changing stucture of steroids,so more of them get out faster.