They're different builds. As far as I can tell, the openjdk builds by Debian for example are not and will not be TCK certified (due to a licensing problem if I understood correctly), whereas AdoptOpenJDK claims some builds are certified https://adoptopenjdk.net/support.html.

In my case the certification probably won't matter, we've been running with OpenJDK from debian for a long time without problems, but I'm curious to test the OpenJDK builds that use OpenJ9 to see if it can outperform Hotspot on some workloads.