Ebbie...I am not sure you even realize just how basic and important your question is! I have been pondering that for many years, and can see a number of approaches to answering it. Some are cultural/sociological, but some are, I suspect, related to the average amount of testosterone in the leaders of various groups. (We breed dogs for their attitude, etc...a Pit Bull has a 'different' opinion about how aggressive one should be in defending territory than a Cocker Spaniel)

You can also ask "why are some humans superstitious?" and get close to answers about why we have different opinions when confronted with the same data. Some people simply have different attitudes about what passes for 'truth'

To stick an old pun in here: You can lead a horticulture, but you can't make her think