As well as only counting the first 5 tags on a post, tumblr.com/tagged/YOURTAG will only show things that were posted for the first time with YOURTAG attached to them. So if you use e.g. the tag "super individualistic and witty tag about black widow" for all your avengers reblogs, but never put it on original posts, then tumblr.com/tagged/super-individualistic-and-witty-tag-about-black-widow won't show any posts. (But YOURURL.tumblr.com/tagged/YOURTAG will.)

Possibly that wasn't what you were talking about, but it's something that used to confuse me :/