As the sibling pointed out, that would be the per year amount, not per month.
But also it's safe to assume Tarsnap's December profits have been steadily growing over the past 14 years (or at least for some of those years), so the number is probably a bit higher than $275k/year. $275k/year is more like the average profit for the past 14 years. (Even then, that assumes that the profit numbers are roughly the same every month, which may not be the case.)
