Stl file quality problem.

Greetings to all the community!! Have you ever noticed a different in stl quality in a model, when exporting it as an assembly or exporting as a simple part.
Recently realized that whenever trying to export a part as an assembly, somehow the mess of the stl preparation differs than exporting it as a simple part.
And the problem is that the 3d printed is better when exporting as a part🤔.
Software Solidworks 2013.
Is it me or someone out there mention it too?
Thank you very much!!