Given that sellers may use the reported bid value to bill bidders, it seems like this could cause a bidder to be charged more than their actual bid value (i.e. if the value is stochastically rounded up). Has Chrome given any consideration to that potential consequence, and considered whether it'd perhaps be better to always round down to avoid buyers being billed more than their bid value?

The goal is for people to be charged the correct amount, in aggregate. We round up or down probabilistically, in a way that means that summing over many auctions will lead to the right total.

Suppose we were doing this rounding to 2 digits in base 10. Then a bid amount of $0.538 would be rounded to either $0.53 or $0.54 — and it would round up to $0.54 80% of the time and round down to $0.53 20% of the time, so that the average is still $0.538 as desired.

If we took an approach like "always round down", then you could game the system by bidding $0.539999999, with more 9's than the person you're bidding against. This sort of "gas station pricing" doesn't help anyone.