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.
Coinbase is always higher than the rest of the markets for some reason... I think it's because they are incredibly slow in transaction times. Right now it's pretty equal tho, look at current Bitstamp and Coinbase prices.
The difference between gdax and almost all the other exchanges are huge, i dont know why there is so much spread between them.