We don't all use tags for the same thing/s.
Some people tag to place threads into particular forums, some people tag so that it will be easier for other people to determine what a thread is about (and that's where there is sometimes the greatest variance - taggers don't necessarily think that a thread reflects what the thread originator has tagged it as), some people tag threads in ways that have meaning only to themselves. There are probably other reasons for tagging as well, but those are the three that come to my mind.
My tags tend to reflect the second and third approaches to tagging. The tags I attach to threads usually reflect what I think a thread is about, or I'm using them for a personal reference (i.e. I'm warning myself that a particular poster might be found in a thread)
I don't believe anyone can change your tags, but your tags might not end up being the ones that end up being the ones visible to the entire site.