The box in the upper right hand corner that summarizes your current search does not parenthesize correctly.
Consider typing the following query in the search box:
([c] or [c++]) [multithreading] is:question. This really does return all 5052 [multithreading] questions that are also tagged either ([c] or [c++]) but the box shows:
Most people would consider this incorrect. The "and" operator takes precedence over the "or" operator.
Now consider the query
[c] [or] ([c++] [multithreading])
This query returns all 95134 questions about either c or (c++ and multithreading), but the box in the upper right hand corner seems to indicate that you asked for the same thing as the previous query.
I can suggest two options: either actually include parenthesis when necessary (the first query above) or flatten into sum-of-products form before displaying.
Note: This bug report grew out of the discussion with respect to the support question I asked here: searching using both "and" and "or" (sum-of-products or product-of-sums).