I just saw this bug now - I think I didn't notice it before because I didn't think it was related to one of my extensions. I changed "Semantic searches" in the title to "Semantic Drilldown" so it'll be clearer.

Anyway, I see the issue - values like "Characteristic subgroup of abelian group" are showing up when searching for "Characteristic subgroup". Yes, if you use the "combo box" input type, it does a substring search, instead of a search on the exact text. I think having it as a substring search is the right approach, since users can type in anything into that field. So, in this case, I'd say you have two basic options: don't use the "combo box" input type, or rename some of the values so that they're not substrings of other values.

By the way, in your explanatory text, "minimized" might be a better word choice than "circumcised".

I'm changing the status of this to "WONTFIX" - I can't think of a way around the problem, other than with the options I listed above.