I think gen ships are generally a lot more likely to have more than one gen ship tagged! After all very few people have only one important relationship of any kind in their life, so once you've started tagging that, you'll often tag more than one. So they turn up a lot without the OTP filters on because there are some & ships that appear in nearly every fic in the fandom, but they are almost never the sole focus.

MCYT is a bit odd on that end, partly because the ongoing lack of useful fandom tags meant people got used to using gen ship tags to filter on which part of the fandom they were interested in, so it's more likely to use gen ship tags in general, and people are more likely to throw on one & tag for that reason even if they aren't tagging much; partly because there isn't really a lot of / shipping in large parts of that fandom so & ships get more prominence.