I disagree with you. In the query "deluge retina", "deluge" is definitely the more important bit. After all you're looking for deluge and are hoping to find it with support for retina. Otherwise you would've written something like "torrent client retina".

Now this is where a bit of guess work comes in but I'd say Google correctly deduces there is no such thing. Even the best result for both terms just talks about some app not supporting retina, and from the looks of it does help with what other client you might want to use if switching to OSX when you were previously a deluge user on Linux (possibly when dealing with retina). But that's not deluge. That's a useless result considering your primary intent was finding deluge. So Google chooses to give you results that might get you what you want over results that (correctly) only disappoint explaining there is no such thing. So in the first three results Google correctly decides that, to get you a relevant result at all it needs to omit the "retina" to get you results that might possibly get you what you want - deluge despite not having retina support - over just giving you the results that are relevant but definitely won't get you what you want. Your query had no results that would get you what you wanted so Google tried to alter the query and see if it could get you something useful anyway.

I think Google trying to give you results that might be hits instead of giving you disappointment in the first place is very sensible behavior.