The only explanation I could think of - assuming, of course, Donald's subbing was on the up and up - was that it was one of those "we don't know how long Bert is going to take with X" situations: he get a phone call and doesn't know if his next 6 minutes or 60 are going to be in disarray... we've all been there. And Bert was apparently sharing the story during the commercial breaks with the audience, as he alluded to throughout the episode. He's behavior makes it seem legitimate to me at least - ha.
(And yes, I'm well aware of the ridiculousness of this speculation, but whatever... it's fun. Ha.)