They changed their versioning process to align with Chrome. Chrome 50 == v8 5.0, Chrome 51 == v8 5.1, etc.
Major versions make more sense when you create a release based on the content. V8 is creating its releases based on time (~6 week cadence). So a 'major' version is nothing special anymore. In 4.9 we greatly expanded our ES2015 support which was huge but only incremented the minor version because of the same reason.
Disclaimer: I work on the V8 team.
Edit: By the way, sorry to derail the conversation. You guys are doing great work! I'm always excited about V8 updates because they generally mean great perf/feature improvements to both the most popular browsers and my runtime of choice, NodeJS.