I don't know about impossible but it certainly takes a long time for everything to iron out.
Right now I'm running good. The past 8500 hands I'm winning at 22BB/100. My all in-adjusted BB/100 is 13 so I've had at least 9BB/100 of good luck for every 100 hands. If it was 9BB/100 of bad luck I'd have won just 4BB/100 instead over the same number of hands. A swing of 18BB/100 one way or the other in that time. It's crazy!
That's just for all-ins, I could have been subject to good fortune in general play too. That 22BB could realisticaly be 0BB, or even a loss.
It's a small sample but the point stands. I think we need a few hundred thousand hands to really know if we're good/bad, lucky/unlucky.
This is very interesting: