My head is getting tangled looking at frame sizes.

Look at this guide:
http://www.evanscycles.com/help/bike-sizing#hybrid

For a 6 foot bloke the size suggested is 19-20in or 55-57cm. But those measurements aren't equal - 20 inches is less than 51cm.

This one is even weirder:
http://www.cycleexperience.com/getting_the_right_size.php

It suggests 17in or 55-57cm, which are absolutely nothing like the same length. 17 inches is only 43cm.

Am I missing something obvious here?

Ta x