I can't give you the source I got it from, as it is something I have read multiple times over the years from different sources. A quick search turns up a lot of articles on the subject, with the claim that centaur teams are better.[0]

I cannot follow you on the unintuitive part. If you have a chess program playing against itself, as long as it isn't capable of playing a perfect game every time, the program being supervised by a human should better, as the human will have a non-zero chance of spotting a mistake (or a better move) with enough plays. With low of the risk of mistakes from the weaker human, as she can confer with the judgement of the program. If the risk of human error is lower than the chance of human insight, the centaur team should have the edge on the lone computer.

After checking up on the subject again, it seems like the effect is no longer strong enough to topple the best chess computers though.[1]

[0]https://www.bloomreach.com/en/resources/blogs/2014/12/centau...; http://www.huffingtonpost.com/mike-cassidy/centaur-chess-sho...; http://www.glassbeans.com/blog/why-centaurs-will-dominate-th...

[1]https://chess.stackexchange.com/questions/15772/how-do-the-b...