Michael is right on this one. Look at the picture. When you use that snap ring on the outside of a shaft, only a small portion of the ring is acually is contact with the shaft - the two "ears" and a bit of the circumference on the side opposite the "ears". That ring is designed for internal use so that the entire circumference is in contact with the a shaft.

On an external snap ring, the ears are on the opposite side so that the circumference of the ring is in contact with the groove all the way around.

It could be that when the previous owner had routine maintence done that the mechanic used the wrong part.