Peope who leave gigs early.
Sure there could be one or two people who think the band suck and just go home, or who were only their for the support, or live somewhere with a bit of a shitty transport service and need to make the last train or something. I've done all of those things, albeit less than 10 times total.
But people who watch the entire show, then leave before/during the encore/last song. For a lot of gigs I do often make my way to the back (or wherever the exit is) so that I can make a quick getaway as soon as the final song is done, but last night I did that at Paradise Lost and easily 200-250 turned round and walked out as soon as they started playing the last song. And given that there were only about 1000 people there, that's a pretty significant proportion.
11/4 Robin Trower
3/5 Maiden United
7/5 One Machine