A well hit putt can still move off line in real life.
To hit wayward shots with woods and irons, you generally have to make a bad swing. You don't make a good swing, make contact with the ball with a square club face and from the middle of the club and then see the ball shoot off 45 degrees right. You have to have made some error in the swing to cause this. Make a bad swing with 3C, RTSM, RTSC or Mouse swing and you get penalised, depending on skill level and assists enabled.
With putting you can hole a putt with a bad stroke because of how the ball interacts with the surface of the green. You can miss a putt with a good stroke for the same reason. Realistic and Bumpy are ways for PP to try and replicate it in the game. It is an option.
If you don't like the randomness, stick to Perfect greens.