Perhaps it is do to with red blood cells, I do not know. I would, however, like to propose a hormonal aetiology.
In order to get cortisol flowing through the body, the cascade is as follows:
Hypothalamus releases Corticotropin Releasing Hormone (CRH) --> Adrenocorticotropic hormone (ACTH) from the pituitary --> Pro-opiomelanocortin (POMC) --> Cortisol from the adrenals
POMC has another pathway that it can follow, which is through the melanocortin pathway. This eventually leads to stimulation of melanocytes (pigment cells in the skin responsible for tanning).
Perhaps raising testosterone has somehow, indirectly inhibited the conversion of POMC to cortisol? We are all aware that testosterone inhibits cortisol directly at the cortisol receptor, but perhaps it has other mechanisms.
Of course, it would be retarded to go around quoting this as the cause of tanning whilst on cycle, I am merely speculating on something which I imagine is more likely to be correct than simply RBC's.