I had never noticed this difference. I do not know if it is an accident, bug or by design.
Recently I installed the AKL (Alternative KiCad Library) from the Plugin & Content manager. It seems to be a quite big and high quality library, but it is making searches more difficult because there are now so many things to sort though and list. It looks like KiCad’s library system is going to need some overhaul in the future. I do not know what that would look like though.
When KiCad V8 was just released, there was a bug in that multiple keywords were treated as an “or” function. If you typed in more keywords, you got more results. I reported that and it was fixed in KiCad V8.0.1 #16974
This makes it plausible that this is also unintended behavior, but I’m not sure.