As was mentioned above, this tranny along with most modern electronic trannies have an adaptive learning capability. They'll learn how you drive your vehicle, and base the shift strategies accordingly. If you baby it, it'll learn a smooth soft shift, but if you're working it harder, it learns a more firm quick gearshift.
You can lots of times experience this as it re-learns after a battery disconnect...It takes some time and can shift a little wierd till it figures out how you're driving it.
When I pulled a car dolly loaded with a car on it, my tranny firmed up nicely (almost too firm once I was unloaded!), and stayed nice for quite some time. More city driving and light throttle openings have since made it soften up a bit, but once in a while, a good hard flogging on a backcountry road will get it back to the way I like it again.
The answer may be that simple...Hills require more load on the drivetrain than flat roads, and the vehicle may have just learned a little harder shift strategy because of it.