This is because those packages will become OpenJDK 11 when that is released. See this mailing list post:
On behalf of the Ubuntu Foundations Team, I am requesting an SRU
exception for OpenJDK. Our plan is to release OpenJDK 10 as the
default JRE/JDK 1 for Bionic, and then move the default JRE/JDK in
main to OpenJDK 11 in September/October 2018 as an SRU.
= Proposed Plan =
Bionic will be released with OpenJDK 10 as the default JRE/JDK and
OpenJDK 11 will replace it once it reaches GA.
If we are going to switch to OpenJDK 11 in bionic once released, we
want to avoid OpenJDK 8 as the default JRE/JDK in Bionic at release
time because any additional interface delta that exists between 8 and
11 not only exposes the archive to breakage, it also exposes external
consumers of the JDK to breakage. In comparison, the interface delta
between OpenJDK 10 and OpenJDK 11 is expected to be fairly small,
especially in comparison with the delta between OpenJDK 8 and OpenJDK
9 that we already know is large. We should therefore release with
OpenJDK 10 as the default JDK in 18.04, transitioning to OpenJDK 11
when it is released.
That's also why the source package of
openjdk-11-* is actually called
A bug report has been filed - please subscribe to it (but don't comment unless you have new information to add, which is unlikely). An Ubuntu dev, Jeremy Bicha, has responded:
The intent is to upgrade openjdk-11 to 11 in Ubuntu 18.04 LTS. That's
why it was named that way. It is a major change and will take some
time to prepare. There is no need to ask for it to be done since it
will be done.
Ubuntu 18.04 LTS was released several months before OpenJDK 11 was
released so it wasn't possible to provide OpenJDK 11 before Ubuntu
18.04 LTS was released.
Sorry for the inconvenience.
Please don't ask for an ETA. That sends out an email to everyone
subscribed to this bug and does not help fix the bug.
This bug is being worked on. openjdk 11.0.1 was updated in 18.10 and
there was a security update that needed to be handled quickly in 18.04
LTS. 11.0.1 will come to 18.04 LTS when it's ready but it is a major
change that needs careful coordination.
So, no ETA at present.