Disclaimer: I know none of these numbers...
Let's say that GV had the opportunity to sell to SoftBank at a valuation of 50B. Let's just guess that they would getting a 500X return. Maybe they invested 1M early at a 10M valuation and got diluted to ~1% ownership. So 1% of 50B is 500M on a 1M investment.
This is a great return for them. Congrats to all involved.
But this could be considered something of a low-end valuation. Maybe it's more... maybe Uber can IPO at a 100B valuation. The difference between a 500X and 1000X exit is huge, but risky. Maybe they end up IPO'ing at only a 40B valuation. GV would still have a 400X exit. This is the risk GV would have to gauge.
Now, if you're one of the later investors and spent 100M for 1%, then the (respective) numbers would be 5X (500M for a 100M investment), 10X, and 4X. For an late round investment like this, you may chose to exit at a locked in 5X, deeming the risk of a 4X exit too great.
Thus my comment that earlier round investors would be less sensitive to whatever the ultimate IPO valuation is. A 400X exit would make a fund. A 4X exit, while good, isn't great.
Again, I don't know the numbers and I suspect they are all significantly higher. I picked numbers that made the math easier. I also don't know who ultimately ended up cashing out (or when). So really, this is all speculation. But if anyone has the numbers, I'd love to know them!