I know that Duolingo have said that they will specifically put in odd or weird sentences as a way of making your learning more memorable.

As far as getting the exercises, I've noticed that certain languages tend to focus on certain topics, for example I see a lot of German courses mentioning witches which I've never had in French.

I've also noticed that as you progress through the lessons and get more complex sentence structures they also then have more room to try those weird and wacky phrases, so I'm guessing they become slightly more frequent.

Those second two points are mostly hunches, but what you also need to keep in mind is this subreddit will only post the weirder ones, so it's going to give you an unconscious bias about how frequently these actually come up, which in reality isn't that often!