The reason UFO's are often described as flying saucers seems to be due to Kenneth Arnold's claim in 1947 to an editor of the Eastern Oregonian that he'd seen nine round aircraft near Mount Ranier which were flying erratically at incredible speeds and that reminded him of saucers skimming over water.
The fact that so many UFO sightings and the aliens allegedly controlling those UFOs conform to rather standard depictions is taken by some as evidence that the observers are not mistaken. They're all seeing the same thing. It is more likely that they see or say they see what they do because of their own and our expectations based on the stereotypes created largely by the mass media. In this respect, and maybe some others as well, UFO sightings might be compared to Santa Claus sightings.
See related entries on alien abductions, cattle mutilations, crop circles, Santa Claus, science fiction and UFOs and ETs.
suggested reading
Gardner, Martin. Fads and Fallacies in the Name of Science (New York: Dover Publications, Inc., 1957), ch 5.