What's the deal with EPO dosing?

Normal hemoglobin values for men are specified as anything in the range of 140 to 170 (14 to 17 on the alternative scale sometimes used). Anything under 140 for men is diagnostic of anemia and, as every standard medical textbook says, "requires immediate treatment."

Now as we all know, when the red blood cell count is elevated artificially by EPO, it is considered unsafe to raise it above the 110-120 range, because anything higher greatly increases the danger of stroke and a whole variety of blood clots. Obviously this cannot be due to the actual density of the red blood cells in the serum, because people without renal failure often go around with values in the 160s and their doctors do not order them bled to bring the value back down to a 'safe' level to avoid clots. Before renal failure at annual check-ups my hemoglobin was often around 163 to 165, and no one ever expressed any concern, but now my nephrologists panic when my hemoglobin goes up to 120.

From everything I have read, the problem is not the absolute level of the red blood cell count, but the amount of EPO which has to be taken to get the patient there, since the chemical nature of EPO is itself what is causing the clotting and narrowing of the arteries. But this doesn't make sense with respect to clinical practice, since if a patient has a normal red blood cell level of 80, the nephrologists are perfectly ready to give enough EPO to elevate it by 40 units to 120, but if the patient has a normal value of 100, the nephrologists are unwilling to give enough EPO to elevate it more than 20 units to 120. So why do they treat the endproduct of the EPO dose at 120 as the marker of the limit, rather than taking the amount of EPO they have to give as the limit?