git submodule checks out the same commit

Question

After I git submodule update it always checks out the same commit. for example 34561.

I do git checkout master for submodule a

Answer 1

I mean why it decided to point to that commit and not another?

Because a submodule always records a fixed SHA1 commit in the parent repo as a gitlink (a special entry in the index).
That is why a submodule is always restored as a detached HEAD branch

You can configure a submodule to follow a branch

cd /path/to/your/parent/repo
git config -f .gitmodules submodule..branch 

The submodule would still be restored to a fixed commit, but can then be updated with:

git submodule update --remote

Make sure to add and commit the new gitlink in the parent repo (since updating a submodule to the latest of a branch would change its SHA1, recorded in the parent repo as a gitlink).
If you don't, you will find back your submodule to its previous state at then next git submodule update --init.

See more at "Git submodules: Specify a branch/tag".