It is indeed just a roll stability effect as was noted. The Skylalne 62 being a high wing airplane has a higher roll stability upright than when inverted.

If you give it a roll control (keep it constant throughout the roll) the airplane starts to roll against this stability, accelerating until a maximum roll rate is achieved upright. This occurs probably during the first quarter of the roll. As the roll goes past the first 90 degrees the roll stability is much less and allows the roll to accelerate more and the roll rate increases. That higher rate will be there between the 180 and 270 degree roll angle, until the airplane goes back toward the upright flight, when the increased toll stability will tend to slow the roll down some. If you hold the aileron in you'll find the roll rate will oscillate.

If you have a high wing airplane with a whole lot of dihedral you could see the effect even more.

Ben