I would guess cross trading, though I'm not 100% sure brokers do/are allowed to do that. Hedge funds do, but I wouldn't think they published stats on it. It's

I would guess cross trading, though I'm not 100% sure brokers do/are allowed to do that. Hedge funds do, but I wouldn't think they published stats on it. It's basically when an entity has opposing orders (like a buy and a sell) that can fulfill each other. Rather than hitting an exchange, they can just update their internal recordings of who owns what.