Again, it's sampling.  An employee could be going from a single business that was sampled to a single that wasn't sampled.  Or a multi that wasn't sampled to a single that was sampled.  The very nature of sampling is that you only count a percentage of them and extrapolate to the whole.

Unless there's some evidence to suggest that a corporate owned McDonalds or Chilis acts different in this regard than a franchised McDonalds or Chilis, or than a single independent restaurant, then the complaint about single vs multi rings hollow.  There's a thousand things that make one store a bit different than another, of course, but I see no reason to say that single vs multi is a major game-changer compared to any other difference.