Why is the plaque marking Anne’s execution in the wrong spot?And where is the real spot?

It is in the spot where the Victorians erected the original memorial but I don't know why they picked that spot. We know from contemporary descriptions that Anne Boleyn was executed on a scaffold built on the area between the White Tower and the Waterloo Barracks (where the Crown Jewels are housed). See http://www.theanneboleynfiles.com/7228/anne-boleyns-execution-site/ for more information and also http://www.theanneboleynfiles.com/resources/anne-boleyn-places/the-tower-of-london-in-annes-life/ for maps showing where she was imprisoned, as well as executed.