It doesn't bother you that your method counts only unique hash values tested? Or that this value is being used on the site to talk about power expenditure (which is clearly related to total hashes per second, not unique hashes)?

You're right that you can compute the N-second average rate easily. But nobody reports the "network hash rate" as such. And since the denominator matters a lot in determining what the number means, pretty much all the numbers you find out in the world are (1) misreported and (2) bogus.