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That value is, if you are using the Windows version at least, reported to be the displayed value of coins in you account.
If you get further coins, they 'fly up' into the top of the screen as usual, showing that they are collected, but the displayed value in your account, of coins collected, doesn't change, when the balance is 2 billion or higher. But if you spend the over-the-displayed amount, reportedly, the displayed amount does not get smaller, until you have spent all of the amount over-2-billion-minus-1, whereupon the displayed value changes downward as appropriate once again.
This is not the very word, but only reportedly, and only for the Windows version. With other versions, collecting may perhaps stop.
Tell me what version, on what playing platform / machine (i.e. Apple, Android handheld, iOS/iDevice/Apple handheld) you are using, and I will look for information specific to that version, and inform you of what I find out. Thanks.
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