Git push to upstream fails with different remote branch name

Question

I have created a local branch

(master)$git checkout -b om-dev
(om-dev)$git branch -u origin/dev 

Fig1: Show the tracking connection of local 'om-dev' to remote 'dev' from origin

(om-dev)$git branch -vv

Fig2: Confirm that my local branch is tracking remote correctly

I have some local commits on 'om-dev' that I want to push to 'dev'

(om-dev)$git status

Fig3: Show local branch is ahead of remote

Now, issue comes here:

(om-dev)$git push

Here, I expect to push my commit from om-dev branch to remote 'dev' branch without using full syntax ($git push origin dev or, $git push -u origin dev) But, I am seeing following complaint:

Fig4: Complain pushing local branch, even after successful upstream set already

Question: Why do I again need to track my branch using -u in my push statement? Shouldn't logically after already setting upstream, it should be able to resolve the remote branch and source when I only use 'git push' command.

Note: I have already tried using -u option, and it works as expected. But I am expecting why it doesn't work this way? The git version being used is: 2.19.1.windows.1