Not only is the Bitcoin price constantly changing, the market cap of Bitcoin is constantly changing as well. Here is a chart that displays the current Bitcoin market cap as well as the market cap over the previous 12 months:
The total number of Bitcoins that will ever be created is 21 million. Every day, Bitcoin mining creats new Bitcoins, so even if the Bitcoin price remains steady, the Bitcoin market cap will rise each day because there are more Bitcoins.
In simplified terms, if there are 50 Bitcoins and each Bitcoin is worth $10, the market cap would be $500 (50 Bitcoins x $10 each). If the next day there are 51 Bitcoins, and the price remains the same ($10), the the market cap would rise to $510, even though the price remained the same.
So the Bitcoin market cap changes dues to both the change in the value of Bitcoin, as well as the change in the total number of Bitcoins.