I don't really have a theory, just sharing my experience growing up being taught to use two spaces, then in the early 2000s consciously adjusting my writing style back to one space, then having my plaintext emails still end up with two spaces anyway.

It's interesting that a document written in a WYSIWYG word processor would have two spaces because I think what originally got me to switch to one was word processor auto corrects removing the extra space, or at least putting blue squigglies in during the grammar check.

I guess my feeling is that although this might be an indicator of authorship, it's not necessarily a smoking gun one way or the other.