What might be causing this?
Nothing is causing it.
0.4 BTC will always remain 0.4 BTC regardless of Bitcoin's current price in USD. If you want to measure the actual change in value of your BTC in your Coinbase Pro account with the current bitcoin price changes, then I suggest you base on fiat values like USD or Euros.
You change to achieve this by using portfolio managers like Blockfolio, Delta cryptocurrency app etc.