While Jeremy is talking in terms of dollars in the presentation, the software is actually keeping balances in terms of bitcoins. You can see on the screen below

While Jeremy is talking in terms of dollars in the presentation, the software is actually keeping balances in terms of bitcoins. You can see on the screen below the two green add/withdraw funds buttons a bitcoin price ticker, e.g. 1 BTC = $446.87 at the start of the video. As he is talking and doing transactions this exchange rate is being updated, maybe as often as every 30 seconds. And you can see even when he's not doing anything his dollar balance is fluctuating to reflect the changes (it goes from $10.00 to $10.05 to $10.01 at the start of the video without him doing anything). But the number of bitcoins remains constant. So that's a fault of the video I think, that he doesn't explain this clearly and talks too much in terms of dollars rather than bitcoins. They'll need to present this more clearly for their actual user video.