Because these are small, mostly visual snip-its of information, they're easier memeplexes to examine than say, a religious belief system or legal structure. Several doctoral theses could be flown under the meme-cat banner.
Take for instance the "Invisible Positive Pregnancy Test". It has wide appeal as the cat's "reaction" could be interpreted as excitement or shock. Even people who haven't personally experienced a moment like this can reference the cultural narrative of "the positive pregnancy test". Because it doesn't encode bits of a larger memeplex (religious or political overtones) it has the ability to appeal to a wide audience. The mirror neurons are firing.
"Whoever Said You Were Adopted" also anthropomorphizes the animals shown. It references the media coverage of inter-species bonding (check these weird stories out) and appeals to our affection for cuteness. Again, even if someone hasn't experienced this personally, they can reference the larger, cultural narrative about it. It integrates with widely recognized cultural references and appeals to our emotional drives. Success!
"Acute Kitty" doesn't anthropomorphize. It shows the contortionist antics of a cat which might appeal to cat lovers as well as people who have no idea a cat is this flexible. The caption also references math humor and corny humor, two sub-genres of jokes. So "Acute Kitty" is more subtle with it's emotional integration, more dependent on the cultural references than the immediate emotional response of the viewer. Wouldn't it be interesting to compare
"Acute Kitty" 's spread to the two previous memes?
"Gathering Wheat for The Motherland" references political history, the propaganda posters of social-realism and triggers the polarized emotions of the "should people put costumes on pets" debate.
"We Need to Find You a Boyfriend" references the urban-legend of women who are too obsessed with their cats to maintain a romantic relationship. This puts a fun twist on it, suggesting the cats aren't enjoying this relationship. It also triggers the emotions around the "costume" debate.
Clearly, to make a successful cat-meme (or any meme for that matter) references to existing memeplexes as well as integration with a hosts' (propagators') emotions is key.
Notice such balance and integration need not be thought-out. Someone may come up with a meme-cat which appeals to them & since they share certain cultural narratives and mental processes with others, the meme is able to spread. In this way, memes mutate and propagate without the need of conscious design. Creativity is often an organic process, designed by the culture and shared neurological patterns, not simply individual brilliance and a strategic plan.