Situation is quite simple:
during a special spin, if you spin 2 reels at the same time (i.e.: spin the second one before the first stops), the give the same card...This card is not the same as the reel not spun, but i think (hence i cannot prove it) that if you respin the three reel together, spinning the third before the first stops, you can win a card, giving the three reels the same result...