A centralized exchange can sell to you cheap bitcoins that they do not have.
As long as users do not effectively withdraw their coins, they will not even notice that they are simply sitting on fake bitcoins.
How do we know that centralized exchanges are doing this?
Well, the banking system is currently suppressing the gold price in exactly the same way:
The banks are selling cheap, fake paper gold that they cannot delivery physically. The banks desperately need to suppress the price of gold in order to discourage people from fleeing from paper fiat money into physical gold.
They are obviously executing the same strategy with bitcoin.
As long as users keep storing their fake, paper bitcoins at a centralized exchange, the fiat banking system will be able to suppress the bitcoin price by flooding the market with counterfeit bitcoins.