When I search for "law and human behavior" (with quote marks) on Google, I get only 106,000 hits. When I search for "law and human behaviour" (again, with quote marks) I get 25,200,000 hits.
I don't have any prejudices about the American vs English spelling, but the journal is really called "law and human behavior" (see http://www.springer.com/psychology/psychology+%26+law/journal/10979). Furthermore, a search for the word "behavior" returns about twice as many hits as a search for the word "behaviour".
Best Answer
Perhaps because people are using more English than American spellcheckers and simply accept the correction from “behavior” to “behaviour” (even if the name of the journal is called "law and human behavior")...
If you are searching for a phrase included in quotes, Google results are related to the combination of the words, independently from the hits of the single words contained in it.