Not checking "Show results as messages" yields less results, all other things being the same.
Expected outcome: No change in number of results, only in the way they are displayed.
Reproducing: See screenshots.
On further inspection it looks like checking "Show results as messages" will display every matching post.
NOT checking "Show results as messages" will display only the first result from each thread which contains matches in multiple posts.
Now you may think this is useful for something of which I'm not sure what it would be.
But in any case this clearly violates the Principle of least astonishment (https://en.wikipedia.org/wiki/Principle_of_least_astonishment).
/me completely agrees on the last point
I suggest to rename that option to "Show all results from each topic, incl. full message text"
Given that renaming 'child boards' to 'sub-boards' was incredibly controversial, I'd be highly surprised to see this happen in 2.1 (and won't happen in 2.0 at all)
Well the better solution would be to separate the two options that have been silently combined here:
1. Show results as messages
2. Show all results from each topic
no better than search and calendar works, I'd suggest completely removing to save server resources, space, & trouble.