Does it increase (it doesn't for me) if the first query is:
+xylophone "irish setter"
and the second one is:
+xylophone "irish setter" +truck
An immediately preceding + (for single words) or enclosing in "" (for single or multiple words) requests Google to search literally. I can only suspect that xylophone is the problem because I suppose there may be synonyms for it or close misspellings (if that makes any difference) fairly common.
I am sure you will have read both the basic help and the more elaborate help pages by Google on this whole area.
Google has and I'm sure will continue to evolve. It does nowadays discrimnate between C# and C++ for example but once did not. I also think that it searches beyond literal strings but displays the literal results first.