Coin market cap calculates its BTC price as an average of all BTC to USD and BTC to USDT pairings. Due to the recent collapse of Tethers (USDT) audit, a lot of people have lost confidence in the system and so Tether is actually trading at less than a dollar. This means that you have to spend more Tether to buy a Bitcoin
Coinbase calculates its price purely off its exchange GDAX. People have to spend REAL dollars on BTC and so people are prepared to sell their BTC for less REAL dollars than fake Tether dollars. I dont think there is anything malicious going on. Its just market forces
Anyway, stop worrying about the price and hold your BTC. When the price hits $1million you wont worry about this stuff!