You can thank the BBC for that. They have shared coverage of the afternoon session, so they want Muzz to play then even if most of the adult British population can't watch
Correct me if I'm wrong but I believe 2/3 of the group matches featuring Murray have to be afternoon matches given the rights the BBC attained. Also think that if Murray makes the semi-finals his match will be the afternoon match. Makes sense this year anyway, Murray's last match will probably be Friday evening against Tsonga, a good time for most of the country to watch (or at least those who have Sky).
Not complaining though, got a cheap ticket for tomorrow's afternoon session a few days ago and all my classes at uni have been cancelled tomorrow so a great result for me.