Only the five most popular tags are displayed at any one time. So, if 6 tags have been added by 2 different people, only 5 will show. Any tag used more often will show (in the case of 6 tags, if the 2 people happened to type one tag in common, that one will definitely show), then it's whichever other 4 tags (I don't know if it's first in or if the thread starter gets preference or anything like that).
You can imagine this kind of scenario. Let's say there are 5 people who've tagged a topic. The # after each tag represents how often it was chosen:
italian food 5
pasta primavera 4
Here you can see that the following tags would definitely show up:
However, if another person (a 6th person in this case) adds vegetarian
, it'll have 2 uses so it has a chance of kicking out rigatoni
, which was also used twice.
Does that make sense?