Topic: Is the random image feature truly random?

In one of my sets, I often use the random image feature because I like randomness and variety. However, despite having over 100 images, I seem to always get the same 10 or 15, especially from more popular creators. Some of the images I have never seen at all, and I've used the random button enough times to encounter the image if it were truly random. I don't really believe it is actually completely random, and if it isn't then what variables affect what image is selected?

Presuming no other tags the elasticsearch query for set:<id> order:random (what random post uses internally) should result in something like this:

{ "query": { "function_score": { "random_score": {}, "boost_mode": "replace", "query": { "bool": { "must": [ { "term": { "sets": <id> } }, { "term": { "deleted": false } } ], "must_not": [], "should": [] } } } }, "sort": [ { "_score": "desc" } ], "_source": false, "timeout": "3000ms" }

random_score is documented here: https://www.elastic.co/docs/reference/query-languages/query-dsl/query-dsl-function-score-query#function-random