Newbie query re visual difference between QTDesigner and Windows rendition

After putting off the inevitable for a few years I am now converting my front end written in Excel VBA to PYQT6. All going well, but I notice that creating forms in QTDesigner I need to do a lot of tweaking to get layouts to match the old app. In other words, the appearance in QTDesigner does not match the rendition of the form when I run the code. Apologies if I’m repeating a newbie mistake.

Looks like no one has any thoughts on this!