Am I missing somehing?
It's meant to be an informative chart for interested investors, giving an idea about past performance.
It's not meant to be a detailed calculation for current investors about their investments
Detailed calculations for your individual investments can be found in the investment tab/detail window.
(Go here and click "Your investments detail")
As it's a daily snapshot, the numbers might already be different if you did not start your investment at 0:00 UTC.
In a few hours, a lot of wagering/investing activity can happen, and if you invest at 19UTC the same day, your individual numbers will be different from those shown in the chart.
Another thing not included in the profit chart is investor activity.
I tend to make a simplified example to show how this can influence your profits:
At a 100BTC bankroll, you have 1BTC invested. A player wins 10BTC, leaving you with 0.9BTC invested.
Now a new investor joins and invests 110BTC, bringing the total bankroll to 200BTC. The player from before loses 15BTC this time.
Your investment will now be at 0.9675BTC (profit: -0.0325), while the casinos profit is 5BTC.
As you can see, other investors can influence your profits and make them negative, while the casino as a whole (globally) still has positive numbers (and shows these in the chart).