It's probably either a hill hold system, or the hill is so slight in inclination that your torque converter keeps you in place.
Either way, it's by design. The torque converter is the component that ensures your car will creep forwards on level ground when you release the brake but don't press the throttle. This works for hills that are not steep.
For steep hills, there may be a hill hold system that continues to apply the brakes until you press the throttle enough to start moving the car.
My 2016 Toyota RAV4 hybrid doesn't have a torque converter, but the electric motors emulate a torque converter vehicle so it will creep forwards when on level ground.
There is a hill hold system too in my 2016 Toyota RAV4 hybrid: it applies the brakes until you press the throttle enough.
Anyway, I wouldn't rely on either of the systems holding the car perfectly standstill when stopped for extended periods. For extended periods, you should lightly press the brake. The hill hold system is merely intended to allow you enough time to move your foot from the brake pedal to the throttle.