That footnote seems very unlikely to me. I would guess that there are far more companies valued at $900MM than $1B.
Remember, these valuations are not cash paid for common equity like in the stock market. It's an extrapolated total equity value based on a smaller amount of cash given for a smaller amount of preferred equity that comes with a lot of conditions such as liquidation preference and maybe even preferred dividends or, god forbid, personal founder liability.
The valuation number is really just a bullshit number and meaningless to anyone who doesn't know the entire term sheet.
The same psychology comes into play when pricing anything else. We price things at $1.99 rather than $2.00 because there's an irrational feeling that a price starting with a 1 is much less than a price starting with a 2.
I think the OP is just making the point that a lot of these $1B companies are priced as such because it gives some extra legitimacy, thereby increasing their chance of success and increasing the value of the VC's share. The question is whether this is speculation or whether investing at that price actually makes the company worth that price.