Only recently I began tracking this metric, and I analyzed the whole data set for 2+ years. While I can not confirm that it's "rock solid" over the whole period, I noticed that (for me) it is "very very very solid" since April this year. I'd say the error margin (again, for me) is +/- 2% of the average. This is a predictability that I do not see for any other Adsense metric. All the other metrics seem to fluctuate wildly, often without any reasonable explanation.
I'd say that Google just pays out per "unique human visitor" (UHV), regardless of the number of clicks, i.e. the EPC we are seeing is just the result of the formula
This would explain why EPC fluctuates sometimes wildly while overall revenue seems to be "more solid". Now the question remains, how is "payout per UHV" calculated?