I believe that mission boxes are now based on your character item level. It may even be based on your highest item level character so that your alts can catch up faster once they have the correct item level followers.

I think they did it this way so that people didn't feel the need to go and kill the easiest bosses on heroic over and over again to get mythic loot, but instead have to have full heroic level gear before qualifying for mythic loot. I know back in Highmaul the first week, some guilds ran heroic 2-3 times just so they were guaranteed the Mythic cache next time they got the mission.

Seems fair to me and if it does go off the highest item level character you have then that is awesome as my alts need some help!