Well, the user base of the tools doesn't necessarily reflect the distribution of repository types among projects. There only has to be one project which many are interrested in (say, the Linux kernel, a project which would be of special interrest to users of a Linux distibution such as Debian) using a specific tool for people (and this includes also those who are not activeley participating in the development of the project) to install the client tool. It doesn't mean that they use that tool for their own projects (if any). I, for one, have CVS, svn, git, darcs and brz installed, and a popularity context would have no way of knowing which I use for starting new projects, or moving existing projects to.
Even if we were to infer that the relative number of repositories is changing in favour of git, I don't see how you can draw the conclusion that _most_ projects start with git or move to git. If we were to draw any conclusion from this graph it would be that _most_ projects are shutting down their repositories completely, since the number of users who have any VCS at all has decreased from >=45% to <=20% (the exact figure depends on the overlap caused by people haveing more than one installed), meaning that the number of VCS installations (and by your logic the number of open-source projects) has been cut in half. So _most_ the projects (>50%) must have been discontinued?