It can happen if you have a narrow foot on the reel and a wider slot for it on the rod.... not really sure there is much you can do about it except use another reel with a bigger / wider foot. Maybe a couple bits of cardboard wedged in to tighten the slots ?
Mind you, unless the movement is significant it probably won't make much functional difference in practice, unless you can feel some irritating / distracting movement or wobbling of the reel on the rod handle when casting etc.