User blog comment:Kirkburn/Technical Update: May 11, 2015/@comment-1398253-20150512014836/@comment-452-20150512155007

In other words: any edit which does not appear on Special:WikiActivity

Without even looking at the source, I can tell you that it first retrieves the most recent X edits from the database, and then from that list, excludes what isn't shown.

So, if it retrieves the last 200 edits, and 190 of those are log entries or edits to namespaces which are not displayed, then only 10 will be displayed.

It would be possible to instead set up the database query to retrieve the X most recent edits in applicable namespaces only, but Wikia have chosen to do it this way.

For comparison, when you select to show only certain namespaces on Special:RecentChanges, it still shows the number you request. Which is one of many reasons RecentChanges > WikiActivity.