What?

I totally do no understand commercials like this. Here we have two attractive girls on the phone. Each realizes she is talking to her twin sister. An amusing situation and one which any talented copywriter could certainly come up with a clever line. But instead, they have this nasty pissy comment, "For once in your life you sound very pleasant." What the hell is that supposed to mean? And why would Discover want to end a somewhat charming commercial on such a sour note? Also why would Julie ask Lisa if she  sounded pleasant if her twin can be expected to give a negative comment like that?