Isn't McRib a seasonal? But yeah, my sister and I use that phrase all the time based off the Twitter hashtag. I've got no idea where the actual origin is but we've even got a little jingle for it. (And I've far too often made smart-ass comments about being incinerated because we are evidently on Mercury)
Because the $5 is a promotional price on specific ones. If you get any other sandwich than all bets are off. (Well except for right now. At least where I live February has full selection at promotional price)