I may have missed it in all the comments, but the explanation for the described phenomenon seems obvious to me, having done a variety of literary corpus analyse

I may have missed it in all the comments, but the explanation for the described phenomenon seems obvious to me, having done a variety of literary corpus analyses. People write to express opinions, and opinions will tend to cluster. A corpus will present an opinion space with local maxima at well defined opinions. As a model becomes more closely fitted to the landscape, results are more likely to be near one of these maxima rather than floating between.