I think in general all website metrics are just an approximation. Even "real" sign-ups are sometimes incorrect as some users might sign-up multiple times, bots might sign-up. Maybe conversions/purchases and money earned are the only hard metric, but that also has to take into account refunds, chargebacks (fake card used).<p>That being said, most of the time those approximations are good enough to suggest actionable steps.
Neither does the sum of returning and new users. They at least point out that the definition of "new" being "didn't let us do a cookie" (plus other stuff) explains some of it.<p>I find the whole thing of metrics measured this way, as pretty bogus. I trust their long URL clickthrough tracking. I don't trust much about breadcrumbs and cookies otherwise