That's because Coinbase (GDAX) already hit the $19,000s a week ago, so they didn't start making new ATHs until today when the other exchanges got to that level. Anyway, this always happens at all-time highs. One exchange always lags behind the others and then people start posting threads like these. Usually Bitstamp is the laggard, though. I'm guessing Coinbase has a much smaller premium this time around because Coinbase traders "blew their wad" last time, so to speak, with last week's short squeeze.

Man, I'm soooo jelly of people margin trading BTC this week. My co-worker got injured so I'm putting in 80-hour weeks into Christmas week! No time/energy for trading.