There could also be other things that make it non-reproducible as well.

It sometimes feels like search just returns whatever it has on hand that seems somewhat similar to your search. It may then do an asynchronous request to pull into cache results from a deeper backend search, but by the time the frontend gets them you've already been served your results. Subsequent searches from other people for the same terms would then get better results than you did, so this sort of optimization would on average improve search quality even if quality is poor for the first searcher.

I have no idea if search engines actually do this. But it seems to explain some of their more mysterious behavior, like immediately returning a bunch of results that have nothing to do with your search terms.