I think that a pretty big part of it is the tendency of people to overestimate things. Its not that hard for "little" changes that tend to happen as something progresses up the chain of command and to the media, to cause a big difference between what reports say happened, and what actually happened. In particular I think this is the cause of things like the reports of Chemcial weapons, the Iraqi tank column, the Basra uprising, and the fall of Umm Qasr (This one is really the medias fault, if you listen to offical reports, it wasn't "we have captured all of Umm Qasr" it was "we have captured some key parts of Umm Qasr").
Theres a really lovely example of this in World War II.
How many people have heard the "It would take a million US dead to invade Japan" line? Its total BS. But its evolution is based off something iteresting.
The origional study said basicly it would take 250,000 casualties to invade Japan. This became 250,000 dead. Someone saw "250,000 dead" and decided that if there were that many dead, it wouldnt be hard to say "500,000" casualties. This sort of logic happened a few more times, and boom 250,000 casualties became 1,000,000 dead.
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