Search results incomplete



I’m trying to get tweets about rock in rio so I’m doing searches with “rock in rio” (all, not only relevants). It returns me a set of tweets but I’m very confused because the search doesn’t return any tweet related with rock in rio partners like volkswagen. If I make a search with “rock in rio volkswagen” I find them.

I don’t understand why the complete search of “rock in rio” doesn’t return tweets related with volkswagen too.