> So when hackers stole about 36 percent of the bitcoins at Bitfinex, they didn't just steal 36 percent of each customer's bitcoins: They stole, basically, all of the bitcoins owned by 36 percent of the customers, and none of the bitcoins held by 64 percent of the customers. (Weighting customers by account size; you know what I mean.)
I think that all, or almost all, of Bitfinex's Bitcoins were stolen. Certainly more than 36%! The hackers generated all transactions and then simultaneously broadcast them. There's no reason they wouldn't or couldn't have gone after every last Bitcoin that was accessible to them. Bitfinex did not realize the attack was occurring in time and did not send out any counter-transactions to attempt to spend all of their coins first (realistically they had on the order of seconds to minutes for this). We know that Bitfinex wasn't using cold wallet storage like they should have, instead opting for some multi-sig scheme using BitGo that didn't actually work, so essentially all of their Bitcoins were available to be stolen, and were.
What's happening is that the 36% haircut is across all values in all accounts, including fiat currencies that were on deposit such as USD, as well as other cryptocurrencies that Bitfinex trades such as Ethereum. So Bitfinex lost all of their Bitcoin, which represented 36% of the total value on deposit with them, most of the rest of which was fiat currencies.