BBC article on attendances.
Lists a few factors there, competitive league this season (which it is) and some clubs trialing Friday night matches.
Will be interesting to see if the figures hold up over the next few months. Dark, freezing, rain-soaked January afternoons will lower attendances, and with the inevitable postponements due to waterlogged/frozen pitches, matches on Tuesday nights won't be as well attended either.
I go to Institute matches myself, and attendances compared last season are improved, I'd say above that 12% average too. Promotion helped, but the biggest increase has been caused by the greater travelling support.