Actually, the Popular Tag box on the Articles by tags pages does not sort the posts tags.
Therefor, the first 4 tags are shown, that's all:
Articles by tags
This MR adds a few lines of Liquid to the layout to actually sort the tags by posts count, and print a list of the 6 most used:
So the popular tags are, well, popular for real ;)
This will create a new commit in order to revert the existing changes.