Similarly there are only 21 million bitcoins. But bitcoin banks could hold 210 million bitcoins off the block chain.
A bit on the chain is worth ten in the bank?
If it works this way with USD, why wouldn't it work with BTC?
Edit: virtual BTC will debase the real ones same way as virtual USD debases paper bills, but they will probably have equal value ($100 bill has almost the same value as $100 bank deposit).