the overall thresold for this Keyword Object is 3 (for exmaple)
Now when you receive a mail which contain the words:
weapon and colt or only smith&wesson, this Keyword Search match, because the thresolt is reached (3)
If the mail only contains the word weapon, the thresold is not reached, because the overall match score is only 1, and the mail passes. Does it make sense to you ?