Why Isn't That Printed Placard Ad Changing?

I was on a commuter train last night and saw a 2 foot by 2 foot printed placard promoting an online grocer. A few moments later it caught my eye again and I was surprised that the content hadn't changed.

"Huh? What's going on?", I thought. "Why hasn't the content changed?"

Now I don't know if the design created a subliminal expectation of interactivity. (The ad featured a woman using a computer and the tag line was something to the effect of letting her mouse select her cheese.) I thought I was aware that the placard was a printed and static. 24 hours later I'm still trying to figure out what that was all about.

The ad was clever, so maybe I was curious as to what the next idea was going to be and disappointed when it wasn't displayed. Or maybe a static billboard in an always connected world is now just so out of place it's shocking.

Maybe it's simply a matter of good design doing its job: capturing attention and being memorable regardless of platform.