if you watch really closely, he scoots forward a couple of times. this shows he is tilting forward somehow. he doesn't look as secure on the bench as he could be. perhaps i just prefer just a little bit lower to gain security in seating and also, to sit back further. less of a closeness to the piano with 90 degree angled arms and more of a back more - and longer fluid look.
now, i don't have a sure opinion on this - but what i've noticed is that some people like pogorelich seem to allow the length or stretch of their long arms to allow the muscles/tendons/ligaments to work more smoothly more at 120-140 degree arm. i could be wrong - but it's what works for me, too, now.
also, what it does is allow access for the elbows/arms, if necessary, to cross slightly in front of the stomach - whereas in this position - he is limited by his own elbows to sides.