Oracle Database 18c: why Oracle skipped 6 releases?

“If you’re confused as to why Oracle skipped 6 releases, it may be simpler to regard Oracle Database 18c  as  Oracle Database 12c Release 2 12.2.0.2, where Oracle simply changed the naming to reflect the year in which the product is released.

Oracle Database 18c is the first version of the product to follow a yearly release pattern. “

https://blogs.oracle.com/database/oracle-database-18c-:-now-available-on-the-oracle-cloud-and-oracle-engineered-systems

Oracle DB Support Roadmap 2019