Interesting that clubs like O's, Bees, Oldham, Vale and Notts Co. feature highly - i.e. clubs with lots of bigger clubs round them. This means that to follow a team like that, you've got to be pretty dedicated. Which in turn means you're more likely also to follow the team to away matches.
Whereas teams like Bristol City, Gillingham, Swindon and Colchester are more remote from big city clubs, therefore their home fanbase may contain more 'casuals' i.e. people in the region who just fancy the odd game, possibly accompanying a regular mate/family member? If so, it's maybe unsurprising that such fans don't travel.
Meanwhile, credit must go to Carlisle fans - crap season, miles from anywhere, yet still travel in decent numbers percentage-wise.