[mjevans]

I too tried to test if it was only one pair that was significant. However I grew impatient and didn't try to come up with more tests when I thought I had a sufficient answer to explain my most vexing observation (negative, positive, positive out of combinations involving negative numbers).

The observation to brainstorm for ways of proving that a statement is in fact wrong, and exhausting them, is such an eloquent way of wording the hunt for a negative.