=+At first, the insurance agent wondered whether there was some kind of mistake in this output. He thought the Pythagorean Theorem of Statistics should work for finding the standard deviation of the price differences—in other words, that SD1Local - Online2 = 2SD21Local 2 + SD21Online2.

But when he checked, he found that 21229.28122 + 1256.26722 = 343.864, not 175.663 as given by the software. Tell him where his mistake is.