The bond scammers were, in fact, counterfeiting some real American bonds. But they had $134 billion of them, and only $105 billion of such bonds had been issued. Moreover, if the amount had been real, the criminals would have been the fourth largest foreign holders of U.S. Treasuries after the nations of China, Japan, and Russia. That's just too much to believe.
When Joseph Jett did his "thing" at Kidder Peabody, he apparently bought more of a particular U.S. Treasury issue than existed. Electronic settlement had just begun, and was not totally accurate. So there was a "corner" on this issue, sending its price through the roof.
In the early 1960s, a serial criminal named Tino de Angelis got American Express to "sign off" on warehouse receipts for "salad oil" that was mostly water. Moreover, the amount of salad oil claimed exceeded total U.S. stocks as measured by the Commerce Department. Even so, the fraud was not discovered until deAngelis defaulted, and the banks tried to cash in their (mostly phony) warehouse receipts. American Express basically split the losses with them, to the tune of $60 million, a large sum for the time.
As an (infamous) politician once supposedly said, "If you're going to tell a lie, make it a BIG LIE rather than a small one . It will be more easily believed, and you'll get the bigger bang for your buck." (All right, we're paraphrasing, because the man was actually America's worst enemy in the early 1940s.) The same principle seems to apply for scams.