The cave centipede is apparently a monster (extends from that class) and is in the monster spawn table, but is registered as a creature, so when spawning monsters and this mob is selected it adds to the creature cap and will continue to spawn more monsters. If that was the only mob in the table then it would continue to spawn a ton of them. Here is a picture of putting a pig in monster spawn table and removing all others (had over 17k before I stopped it).

Also, minor though, because of this, if you place a centipede egg on spawner cage and you are in peaceful mode then they attempt to spawn, but die and you see poof particles. This doesn't happen with other monsters like zombie.