Disclaimer: I have no idea what estimation theory is about. But looking at estimation-theory I cannot help but think that either the tag means something extremely broad (so broad that I hesitate to think that there is a coherent underlying theory) or that some users are using the tag for things other than "estimation theory".
Perhaps related is the fact that we separately have the tags
Question: What are they and how are they related?
(For example: I have some vague idea about what approximation theory is. Some of the questions currently tagged under that tag have absolutely no relation to the description on Wikipedia. I suspect we either have to dump a bunch into approximation or create some new tags. Absent a good tag wiki, I think quite a few users just tag both (approximation) and (approximation-theory) when they don't know any better.)