As far as I know they really are just numbers in databases, but if it's foreign currency, they can't just magic them out of the DBA's fingers. There probably are ledgers that have to agree with each other, e.g. if a Russian bank claimed it had those 100 Trillion dollars from a Cypriot bank, then the Cypriot bank would have to confirm it gave them that much, and then show everyone the source of that money.

Which does make one wonder how easy it'd be for your trick to work.

North Korea is a major printer of fake $100 bills, a piece of paper that's actually worthless but others are willing to trade $100 worth of goods and services, or exchange for a bank transfer. Until a bill is found to be fake and destroyed, I guess there really is $100 more in circulation, and when the last person found out the note is fake, in effect NK has robbed them out of $100.