My crypto prof in uni commented on this. (This is from memory). Apparently, the number of rounds (16) seemed to have been chosen arbitrarily. Many years later, when attacks on Feistel(?) ciphers (of which DES is one) became public, it turned out that these attacks only worked up to .... 15 rounds! So the speculation was that the NSA knew about the attack, and chose to set the round number <i>just</i> out of reach...