Judging from the responses so far I'd say it's a little of both.  I
also thought of another angle that may have some bearing on this and
that is the rider's attitude/demeanor.  My older mare is extremely
competitive; we've been riding together for ten years now.  In ten
year's time I have become less competitive (read controlling) myself.
Perhaps my little mellow mare simply reflects my own personal growth
in that area.  I instilled the competitive edge in the older mare
years ago, now I am conveying a more conservative way of going to
the young mare.  My animals have tended to live up to my expectations
whether I expected good behaviour or bad behaviour.  They never seem
to let me down!  
The bottom line is that I'm lucky to have a mare like Cassidy.  Over
the long haul she will probably do very well.
Joyce