That's true, but let's make it just slightly more complex: Our contract is to swap 10 EUR for whatever it is worth tomorrow in USD according to what "bloomberg.

That's true, but let's make it just slightly more complex: Our contract is to swap 10 EUR for whatever it is worth tomorrow in USD according to what "bloomberg.com" gives as the representative rate at 12:34pm. Now, this is easily codified, but ... a) Someone could hijack the "bloomberg dns" b) Someone could hijack bloomberg's BGP both the above would likely be defended by using https, but CAs have been hacked before, and there are questionable state controlled ones. c) Someone could hack inside Bloomberg itself to give the wrong rate d) Bloomberg might have a bug that is struck exactly at 12:34pm on that day, which shows the wrong rate. All these 4 things may happen independently of our contract - Bloomberg is not a side to our agreement, and other actors might have reasons to try to make it give wrong data. In "The real world", if the rate is really off (say, half what you expect), you'd look at Thomson/Reuters and Yahoo, realize Bloomberg is borked, and react accordingly - and a court would likely support that court of action. This is the kind of wiggle room you want, because in the real world, it is impossible to lock down all of these improbable, weird, but possible, outcomes. Contracts that are so simple that there's no potential need for wiggle room aren't very useful. In real world contracts, the more money is involved, the more both sides try to protect themselves against such weirdness; and yet, courts are sometimes involved, either because interpretation is disputed (comparable to "code has underfined behaviour") or due to events not covered in the original contract.