Perhaps it has to do with the amount of money that's being invested in these late rounds. When you're doing 10k angel rounds, you spread the money around and hope one of the companies take off. When you're investing tens of millions of dollars at nine and ten figure valuations, you do so more conservatively in a way that is more likely to guarantee a return. They're not saying they don't believe in the companies at this point - in fact, you can almost argue that they're saying these late-stage companies are definitely good for the money - but more so that the companies at this point are less likely to make astronomical gains.
