I remember seeing that one in Latest. I don't know the answers as to why some memes take longer to feature than others.
It seems that the way they do it is that images on non-standard templates always take longer, presumably because they need human eyeballs on them (presumably to make sure there isn't anything pornographic in the image).
It might be that there are different kinds of mods on different shifts, and some mods seem more confident about releasing things, whereas some mods might bump the decision up to higher-ranking mods if they aren't sure, which is why some seem to take a lot longer. Or it might be that the only review memes at certain pre-set times of day, and so the time it takes for your image to feature will depend on how close to one of those times it is.