It's the only criteria that makes that lineup sensible. Saying Xavi is the best midfielder out there is looking only at certain aspects of midfield play, same when ranking Keane second.
Masopust is by all accounts the best all-round midfielder out of that lot and you are questioning whether he is better than Gerrard...
There is barely any footage of Masopust, all I know is what I've read online and its slim pickings. To say he's better than either Keane or Xavi is insanity
If you look at the ranking I had him better than Gerrard, the reason a ? is there is because I truly don't know much about him, bar reading some stuff online. In all truth he probably wasn't better than Gerrard but that's another debate that has been done to death.
The ranking was based on just their overall ability as players, the same way you rank any player and say that Nesta is better than Vidic or Ronaldo is better than Drogba etc. Obviously there will be aspects of play in which the worse player is better than the better one, so in those two cases heading for example, or tackling when it comes to Xavi.
Doesn't change Xavi being overall the best of the bunch. He's definitely on the slide right now which isn't great timing for me but once hes retired he will be looked on as one of the games greatest ever midfielders and best ever passer.