The chain angle depend on distance from the jockey pulley to the sprocket.
If the jockey pulley is too far from the freewheel sprocket, you have to shift your derailleur farther to create an angle sharp enough to make the chain climb out of its comfortable bed. When it finally does, the chain probably will overshoot and shift two gears when you only meant to shift one. This is because the derailleur cage has moved farther than the normal 5.5- to 6-millimeter distance from one sprocket plane to the next. Once the chain is derailed, it is just as easy for it to jump two gears as one.