I'd have to test this to be sure, but I've always assumed that the window would scroll until the selection was in view not until the selection was vertically centered. That could place the selection at the very bottom or top of the screen if the selection is not already in view.
So maybe the field with the focus was already in view the time that you found it centered.
I checked FileMaker Help and don't find anything there that would lead me to expect the selection to automatically be centered on the page.
There are a number of posts here in the forum where the following method is discussed as a Kludge that sort of centers the window on the selected field:
Determine whether you need to scroll up or scroll down. If scrolling up, put the focus a half window above the item you want centered in the window, scroll to it and then return focus to that item. If you are going to be scrolling down, do the opposite.