Ive had this too, I think the fact that it happens when your having a "quiet" night n not really trying to pick up girls. This makes the approach harder for them so they use this to try n break the ice.
In the past ive played along abit n said stuff like "make me laugh then! GO!" or "Are you the fun then ?
" then some cocky funny banter.Then go for her number and say "ok well ive had fun tonight, if i ever have another boring night i wanna give you call, im gonna put you in my phone as TheFun"
Dunno, works for me just gotta say it all in the right tone, also its a light easy way of gettin a number close.