Does seem a bit off, but how big/busy is this bar? Do you really believe that 60 or so folk heard them there and then didn't come to your concert. It would surprise me if that many of your intended audience were in one pub in town. I suspect they were going to a loss maker for your art centre anyway. Also worth considering is whether they believed they'd cleared it with the art centre first by asking for a suggested venue to do a gig.

Scott