Q: Cloning - source point problems

I'm having problems with cloning as far as selecting a source point. I option-click over the area I want to clone from but when I use the brush to clone it works for part of the beginning part of the brush stroke but then clones from pixels I haven't selected. For example, I'm trying to eliminate reflections in a flat panel television. I'll clone from the most black section that has no reflections but soon after I brush in the clone on the reflective parts of the television the brush will start painting in something from the nearby shelf on the entertainment center. If instead of brushing in what I'd like to clone I instead do the equivalent of a mouse click it works better, but it's not as smooth and I'm left with obvious circles from the brush. I know I can adjust opacity, but I'd really like to know why I'm having this problem.

I've enlarged and reduced the brush size when selecting the source point - always being sure that the source point only includes the area I want to clone. In my example above I notice that if the distance from the black pixels of the screen to the colored pixels of the shelf is 1", then the brush stroke when I clone changes the pixels used after about this same distance - roughly, give or take.

Does this mean that this is the only way the brushes work and that instead of being able to do one longer brush stroke I need to use several shorter brush strokes?