When you have your foot on the brake, all four wheels are locked. The handbrake only locks your rear wheels. Releasing the brake pedal, especially on a hill, will allow your front wheels to move a miniscule amount down the incline (backwards when facing uphill, forwards when facing downhill) while the back wheels remain motionless. Your suspension allows this as the car finds a new balance with only the back wheels locked by the handbrake.
HOWEVER.. that to me has always just felt like a settling or re-leveling of the car, and not an actual rolling movement. In an automatic car that's been put in park but not had the handbrake engaged, it's normal for a short roll to take place upon release of the brake pedal, but cars really shouldn't roll, meaning the back wheels rotate, with the handbrake engaged.
Do you have any more travel available on your handbrake? If you pull it up harder, say an other click or two, can you get the car to remain stationary when releasing the brake pedal? If so, then your parking brakes need adjustment or new shoes, or you just need to man up and pull on the handbrake harder!
. The adjustment can be done fairly easily. New shoes requires removing the rear calipers and rotors and messing around with fiddly springs which I hate to do. I drove for years w/out a parking brake on my last car just 'cause I hate replacing them so much. It's not that bad, really.. just frustrating if you haven't changed a lot of them.
If you count on your parking brake, then I'd definitely check into your symptoms further. A definite roll, i.e., all four wheels obviously rotating, should not be happening.
Hope that helps..
Barry