As I understand things, the site Search will search for the root word, which is max in this case. I think it's also supposed to search for an exact match if you put the word in quotes, like "maxed", but I also vaguely recall reading something in a thread here about that not working entirely right.
It's possible that the thread you were looking for was in the search results, but much farther down in the results. When I searched for "maxed" in titles only, it showed 25 pages of results, and the thread you wanted was on page 6. Many of the titles had the word "max" in them, not "maxed", so it's plain that search is not treating "maxed" as a request for an exact match.
Anyway, you can add additional words to narrow the search, and I've found that works more reliably than quoting the search term. For example, I searched for max out in titles, and the thread you wanted appeared as the top result.