i think rate of accumulation does play a role. One year I fell 3k short of Silver and they requalified me to Silver... Also when I got ruthlessly dropped to Green from Gold, and ranked up 20k miles in 3mths they made me a silver again (this was 1 J longhaul return + 1 longhaul Economy).

However subsquently I earned 50k miles all in paid J (LHR, NRT, JFK) + paid Y YVR, as a silver, but was not granted gold when renewal came but they allowed me extra 1mth to makeup the short-fall to qualify to gold.

so the bottomline to me is (1) qualification to higher tier is a lot more strict than retaining existing status (2) rate of accumulation is a different formula. (3) higher cabin fares definitely help increase your flexibility for qualification to higher tier in terms of extra-time to make it up.