The most recent build for Linux is indeed 2018a. This build is only updated once a year. We should have another Linux release with 2019a, coming soon.
The version number is sometimes different from the release number because it's based on the year when the build is compiled. In this case, since the work was done late in December 2017, it's tagged "2017". This doesn't affect operations in any way, the version number only exists to facilitate support and debugging.