Those prices are not from coinbase, they are from GDAX, they are the same because they are the same company, but coinbase did not put those rates.
And maybe you mean that "gdax" has always had the most expensive rates, but that is true, the last time that bitcoin had an all time high, gdax showed $19,300, while bittrex/polo were at $17,400 if i am not wrong.
You forgot Bithumb which peaked @ $22500 during the insane run. Idk if they even count that as an ATH since it surpassed all the known highs in bitcoin, but then again, it's not a Western exchange so statistics won't probably count it as one. There are other scenarios in which Asian exchanges surpassed Western exchanges in terms of peak price, for example some Indian exchanges outperforms Western exchanges BTC/USD but still they won't consider it as an ATH lol.
It seems to me that all the rest of the exchanges are about to catch up with the new $20000+ reality. If an exchange crosses $20k, the rest would follow, and it's finally happening. Bitstamp is about to hit $19k. Usually Bitstamp is the most sloppy one to get prices going. Once Bitstamp crosses the barrier, it will be "official" that Bitcoin has crossed $20k, since once Bitstamp and Bitfinex do it, most have done it already (also a lot of people use Bitcoinwisdom to keep track of prices and these are the 2 big volume exchanges there).