I am new to bitbucket and git bash and came to know about this common yet imp feature on bitbucket – sync branch

So if I have, say a branch feature-A checked out from main branch, thus ideally the parent branch of feature-A is ‘main’ branch.
Now in bitbucket you can sync changes from parent branch into the child branch in just one click, and more importantly we do not have to remember what the parent branch actually was.

I was trying to merge release branch into develop branch in bitbucket. For this when i am creating pull request its not allowing me to create one saying develop is already up to date.
But i know for sure there are lot of differences between these two branches ,also i have verified manually that there are differences.