On my friends 2001 V70, the tranny would drop into neutral when stopped in
drive... and then it would engage first gear when you hit the accelerator...
it wasn't the smoothest engagement either, and they fixed it (I don't think
it drops to neutral anymore at all... the software patch they received was
availible in 2002 or 2003... it's been a while since her car has acted like
that, it works beautifully now with over 150K Kms on it) - maybe your XC90
has a similar problem... You're right tho, every automatic i've driven on
hills will either hold - or in the case of of our TDI Golf - actually be
able to pull itself up at idle speed... Never roll back down tho, that would
be very uncomforting.

Anyhow, if your XC90 was in neutral, it would roll backwards I would
imagine... She never said anything about her V70 rolling (and I never felt
it roll), but she is one of those on/off switch style drivers (the DSTC
light flickers almost every time she takes off) with the gas and the brake
so the car wouldn't really have had time to roll.