I used to believe the same thing ( engines being stronger in single PV mode than multi PV ).
But confronted with examples to the contrary I must say that there exist cases in which the correct move can only be found in multi PV mode by an engine.
I presented an example here ( see analysis of the Topalov - Shirov game ):
http://www.chess.com/forum/view/general/positions-engines-get-wrong--please-contribute
Many chess engines can calculate multiple lines at the same time. But the engine will give stronger moves if it only evaluates one, I've heard.
Why is so? What if the engine finds a very good move and analyzes it forever, but it turns out that if the engine had gone, for instance, "depth 32" on another line, that would be better?
So my question being; When calculating 1 line, will the engine exclude all other moves than the first "very good move" it finds? If not, what's the difference between evaluating 1 and multiple lines?