With Lua transclusion I'm referring to the side effect of invoking mw.title:getContent() on any page. Per mw:Extension:Scribunto/Lua reference manual:
getContent(): Returns the (unparsed) content of the page, or nil if there is no page. The page will be recorded as a transclusion.
That is, whenever a Lua module is invoked on page A and retrieves the content of page B using this function, A is recorded as a transclusion in Special:WhatLinksHere/B. Now, if we categorize B in Category:X, CatWatch would notify about this transclusion even if no additional page was added to Category:X: [[B]] and one page added to category (see i18n messages recentchanges-page-added-to-category-bundled and recentchanges-page-removed-from-category-bundled).
It's also possible for a page to retrieve its content with Lua, in that case it references itself on WhatLinksHere. Since there is an use case for this on plwiktionary (see task description in T109638), we are seeing lots of superfluous and one page added to/removed from category bits in log comments with CatWatch enabled (RC).