Why the garment that is from the MD store refits with ease, but custom-made (not from MD store) are problematic?

I am trying to figure out reasons why the garment model that is from the MD store refits with ease, but models that are custom-made (not from MD store) are problematic to refit to the different avatars. Autofit doesn't work smoothly as it may need to do, as it is advertised.

There is a lot of trouble when making a fitting suit for custom avatars. Saving custom fitting suit as an avatar file doesn't help, doesn't work out of the box. Not if I even save it in MD default avatar folder.

I am having more luck adding an avatar as an FBX file and checking the tickbox to "Auto create... fitting suit", and then it works only if the garment model is simulated for a moment, saved as a separate project, then imported in a new project + and then only adding avatar model as .fbx.

And after getting through all that - Autofit - if it works, the result is a very messy-looking clump of polygons and triangles.

What are the reasons that MD store garment can be refitted to a custom avatar easily, but a garment that is created in MD, and then attempted to refit to a different avatar ends up very problematic?

ps: custom garment whit which I am working is created by someone else in MD, and I am just a guy who is trying to refit it to a different avatar model.