On the OTN downloads, they only have 18.4 available for AIX. If you had 18.3, and applied a more recent CPU, your version would change to 18.4 so you'd be getting to that version eventually. See this: https://mikedietrichde.com/2018/11/08/why-release-update-revisions-rur-are-tricky/
Oracle 18.3 is the base 18c version (18.0) with the July 2018 Release Update (RU) applied to it. If you apply the Oct 2018 RU, it will go to version 18.4. The Jan 2019 RU will change it to 18.5 and so on. That blog post has the RU versions with their release month.
What you see on the OTN download page is basically the 18c version with a specific RU applied to it.