I believe there are two reasons for the discrepancy between Buffett (a realist, who's very rich because he's very real) and the U.S. Government (agenda-driven, whatever it takes):
(i) CPI is not the same as increase in the cost of living - let alone increase in the costs of insurance. As in Australia, the items that comprise the CPI basket of goods and services are subject to variability, as determined by the government and for reasons that suit them. Even a small difference in percentage between those two will compound substantially over the 35 year period shown on that graph; and
(ii) The only material difference in the graph over this same period is predominantly in the number / costs attributed to "Severe Storms". Take those green bars away, and, like Buffett, one would hardly notice the difference. Yet "Severe Storms" are definitely insurable events, so for all we know, there may just have been a change to the way that "Severe Storms" in the last decade or so have been classified.
