When a project is relaunched after initial failure to fund, the original project page becomes very hard to reach. You need a perma-url saved before the replacement project is launched. At least if it has the same name as the one that failed to fully fund. I saw this with one of the projects I backed (backed it both times).
Also, projects exist in a sort of limbo between the time they are created and the time they are published. Perhaps some never leave this state. These are only accessible if you happen to know the private url of the unpublished project. Usually these are shared by the project creator within a closed "inner circle" for feedback about their campaign wording/imagery/etc prior to launch. I have seen this on two projects where the creator of a successful project went on to kickstart another project later and asked some of the backers of the first project to take a look and return some feedback before the new one was launched.
Maybe those kinds of things put a dent in the missing 18%?