Your haircut is reduced due to the synthesis of a synthetic call when you're long, and a synthetic put when you're short (futures). SPAN would account for this even though you are obviously not doing so.
That's right destriero. This trading procedure is intended to keep SPAN and Exposure margin as low as possible (small investor). (It is in fact, understandably, quite hard and slow to produce a profit "juggling" only one contract, but the purpose of the experiment is obviously to see if it can be done, statistically, in the long term.)