I was doing some search on Google Scholar and I typed in
MPC | "model predictive control"
this search entry returned around 1.72 million results. Then I typed in
MPC | "model predictive control" | RHPC
and that search result returned around 1.68 million results. I don't understand this behaviour. Why did search in the second case return less results than in the first case? It should have returned equal amount or more results by the logic of OR operator. What is Google doing in the background, is it doing some kind of relevancy search which has precedence over OR operator?