Without being there to see what happened, it would be hard to hazard a guess. If the table rolls off, you should be able to see the object ball curve away from the pocket. Some tables will roll off in different directions depending on where the ball is on the table, so it's possible for the ball to change directions a couple of times on a long slow shot. Obviously, a ball hit harder will be more likely to go in a straight line regardless of whether the table rolls off or not. I have played with balls that are out of round and they will roll off, too. If the ball is out of round, the roll off will be in random directions which is different than an uneven playing surface.
It's good practice to shoot those long slow shots. They do come up during games and are harder to execute than medium or fast speed shots. Accuracy of hit has to be much higher.