My guess is that when you see something like the Washington Monument on a regular basis from a distance, it seems smaller than a tower you've only seen once but got to see close up and even climbed to the top of. Plus, the WM is probably perceived differently as it is a nondescript obelisk rather than a building with floors. Why they thought the leaning tower was so short is odd, but maybe the popularity of foreshortened photographs of people "supporting" it could contribute, especially if they had those photos taken of themselves? Another potential factor is the different width:height ratio between the two structures.