In adding item through top right + icon, you can create just totally a new form with no ability to relate any data as pre-filled data.
But when you tried the other way by creating a form button (or just button with “open form” action) you are able to bring in columns which you made-related, to make it as a pre-filled data.
The question is why? Why can’t both of them act the same way? I think the preferable one supposed be the 2nd one when some data is available.