HO is looking at year-on-year declines in a given calendar month, not month-to-month declines. According to BP Statistical summary, depletion rates where:
99-00: 8.3%
00-01, 7.1%
01-02, 0.5%
02-03, 8.4%
03-04, 10.1%
and the evidence for 04-05 is that it's running closer to 15%. So depletion has been accelerating. I assume the 01-02 anomaly represents a new field ramping up, but I don't know.

You assume in your analysis that depletion before addition of new fields will be 7%, which seems over-optimistic.

Sorry for my mixe-up with year to year and month to month, ill look into it more tomorrow see if i can come up with some predictions and stuff from the industry.