This is how the naming system has worked, and there is a system. A point comes along where the next version is the same name as the verision before, with an extra word, and then the newly added word becomes the link to the next scheme. For example, leopard, and then “snow leopard” with the “big cat” naming scheme, up to “lion” followed with “mountain lion”,which passes the torch to the “mountain” scheme. Now we have “Sierra” going to “High Sierra”, meaning that “High” is likely the connection to the next naming scheme (could be space/sky related).