The original proof of work algorithm used a number of leading binary zeros as the threshold. A hash can be treated like a random number that radically changes whenever the data being hashed changes very little (e.g. with the nonce). Satoshi changed the leading zeros (power of two) comparison to a comparison between the hash and a computed difficulty number between 0 and 2^256. The difficulty is calculated based on the average number of blocks found over a two week period. The "leading zeros" method was not granular enough to maintain 2016 blocks per fortnigh, this is why the hash is compared with and must be less than this non-power-of-two difficulty number.