One is the rating of the novel in that month the other is how many chapters are released by that translator in that month I believe
IIRC. The activity ranking tracks things like new ratings, reviews, reading list bookmarks (updating which chapter you're on), etc. Popular just tracks how many people have the novel on their reading list. Or something like that - I don't really remember. On the novel's page, you can find the two rankings below Release Frequency. The one literally labelled "Reading List" is the Popularity ranking. It'll tell you how many reading lists the novel is on.
Thank you! If its just sheer number of people having that novel on their reading list, shouldn't https://www.novelupdates.com/series/dragon-marked-war-god/ (has 11,297 reading list, but rank 40) have a much higher ranking than https://www.novelupdates.com/series/the-dreamer-in-the-spring-boudoir/ (only 2,648 reading list and rank 38) on the monthly ranking for reading lists?
monthly readling list I remember is about how many people added a novel that month. So dreamer is in less reading lists but more people added it that month to their lists then dragon