Is it a bug, or is it a feature?
Fortunately, the search engine is quite flexible and it is designed to serve the context of what you are looking for; (it used to be a whole different kind of beast;) which is in this case, questions about entropy, but not necessarily containing the word entropy.
This behaviour seems to be a rather silly at first, because asking a question about entropy without using the word entropy is hard; really, really hard.
Usually, the combination of a meaningful title and appropriate tags should give you a good enough clue and should give you relevant information first. That is how most engines are designed for. After all a full text search gives you plenty of results, but are they any good.
The issue is an underlying one, especially in sciences. In our field we tend to ask about the things that have specific terms, which will eventually be part of the question. Therefore plenty of tags will also appear as plain text in the questions or even the titles. It is really hard to avoid that.
The search engine is not really built for chemistry, and the defaults are not made for veteran users. If you use the goggles engine, you'll have no clue how it'll serve you the relevant content; at least here it'll tell you.
Well back to actual content, troubleshooting. I've mentioned before that the engine is flexible. It's features aren't even that hidden. I'll recommend: How do I search?
Here are some of my favourites:
user:me - a list of all the great contributions of the most important user *wink*
title:question - buzzword; honestly though, it is a nice tool if you remember a word of the title
body:wrong - It's always a little bit funny in which contexts this appears.
score:9..9 Looking for some votes to spend …
is:question - all questions, only questions. Use it with the score operator for maximum enjoyment.
"literal" - Safdar mentioned this; not one of my favs, but I include it.
Okay, you'll get the gist.
I have tried wildcards, not a fan though. It works mostly, but offers a few false positives.