Almost anytime a startup gets to a valuation that high, the founders have had an opportunity to take some money off the table. VCs like this, because it keeps the founders' incentives aligned with the investors' (they are more likely to take risks and swing for the fences if they don't have to worry as much about their own personal net worth).

It's dangerous if the founder divests entirely, but you want them to have enough that they aren't too worried about the risk of the company collapsing to make bold decisions when necessary.

That said, I don't know Theranos's cap table, but it's probably unlikely that her current stake in Theranos is worth $1B on paper even today - did she really manage to achieve a $9B valuation while holding onto 11% of her company?