Remember that the number of vulnerabilities that are discovered is not necessarily related to the number of vulnerabilities that exist.

The OS that tops the charts (unsurprisingly) is the most used one.

The ones that come in at the bottom (some weird proprietry versions of UNIX) were the ones with the least deployments.

This does not surprise me, neither security companies nor hackers are going to bother looking for problems in OSs that hardly anybody runs.