I don't think it is a Mac/Windows thing actually. Although, that has also been an area where people have complained about y-axis disorientation. It is the perception that the wheel is actually rolling the screen up and down. The portion that your finger touches is opposite the portion of the "virtual wheel" that touches the screen and moves it up and down. It isn't needed for the volume control because it is a horizontal bar that is increased or decreased. It is personal preference, but feels completely natural to many of us. It actually gives the feeling that the wheel is connected to the screen. Who knows, maybe it only feels "wrong" to a small percentage of users. Maybe there are 5% that feels like it is backwards. Or, maybe it is closer to 50%. Maybe it's 90%. We can assume that it didn't feel wrong to the designers and engineers (at least not the majority) that designed it.