On your vehicle's profile page you can change the filter to display the stats you're looking for.
^Not your vehicle, but you can see it's currently filtering "1 Month". Your's is at "2014" which is why you're seeing 5.651. Click the link and you can change the filter to "All Time" to see the costs of all your fuel-ups since you've begun tracking.
I'd rather see that average fuel price is used to approximate fuel-up costs when the cost parameter is missing. Now it says "total spent" and displays only the cost for the fuel-ups with the cost filled in. My suggestion is to add all fuel-ups using the average price for the ones missing cost parameter. "total spent (approx)" might be a good caption when this is done.
Also I have noticed your calculations goes bananas when "," is used instead of ".". In some languages "," is used as standard and it's sometimes easy to format digits as one is used to. I was amazed to find a V8 Blazer with Prius-MPG until I saw the ","-error.