If you search like "animal food" you'll get a lot of memes related to animals and food without specifically mentioning those words (e.g. "dogs" and "eating" in the caption) so it's definitely using a neural net to grok the semantics.
The trigram thing might just be a weird glitch in the pretrained model we're using (sentence-transformers/paraphrase-distilroberta-base-v1)
not useful for me, since I usually want a meme to match a mood or theme