This, IMO, is the real problem with pg's "data". For example, I'm currently doing a startup. There are over ten people who are currently participating in one way or another. We even call them "founders", thus, when we grow large, pg would count us in the multiple founder column.
But there's really only one founder, and that's me. I'm the only unreplaceable person in the company at this time, I worked for 10 years before I even got to the point of incorporating, and if I left to start something else, the company would immediately fold. I recruited every person, and could unilaterally let anyone go. It's my money and reputation on the line. I wrote the business plan, designed the product, and most importantly, identified the market and invented a way to reach it and fund the company during growth.
In five years, yeah, I'll be able to move on without killing the company. But not now, not when it's still a startup.
Dropbox was the same, and pg nevertheless counts it in the multi-founder column. Oh well, as I discussed in another comment, it's in YC's best interest to promote multi-founder companies even if it has zero effect on the outcome.