The full saying was: "What you lose on the swings you gain on the roundabouts." This was probably a reference to a showground-owner's claim...in other words, on some days the kids would pay to pile onto the swings and provide his income, though the roundabouts were neglected. Obviously, it would be vice versa on other days. Nowadays, of course, it applies to any such 50/50 or up/down situation.