I dont know EXACTLY, but Ive concluded that ranking is determined mostly by the likes you get on your most recently uploaded chapter and slightly by the likes youve gotten on all chapters youve uploaded in the past two weeks. When you upload a new chapter this resets and you climb up the rankings again.
Rating has little to no effect on ranking, as well as subs.
Your rankings do seem a little strange if what youre telling me is accurate, all I can conclude is that your competing series just get boosted by maybe having more frequent updates (so boosting the overall average likes.)
For me webtoons ranking has been consistent, Id always land in an accurate spot every week according to the likes on my recent chapter. But Id also get a lot of likes which also might be more obvious for the system and easier to rank. Idk.