It is unlikely you're going fast enough in a tight spot for a parking space to be experiencing any degree of "understeer"...at least any that would be noticable.
I see you have the AWD, and what you are experiencing is most likely the wheel travel difference between the front wheel and the rear wheel. In any turn, the differential allocates the majority of power to the outside wheel. This is due to the fact that the outside wheel has a longer radius and a further distance to travel than the inside wheel. The differential simply see's this as the least resistance.
On your car, you have two differentials - front and rear. Since the rear wheels basically cut off the angle of the arc of the front wheel travel, the rear wheels are travelling a shorter distance than the front wheels. An exagerated (and somewhat incorrect) example of this is the hypotinuse of a triangle. More specifically, the rear outside wheel is travelling less distance than the front outside wheel. The transfer case that sends the power to the front and rear differentials is unable to compensate for this degree of difference and you feel the symptoms you describe above. This is also the reason that 4WD/AWD vehicles have to have the same size diameter tires on the front and rear.
In most circumstances, you can't notice this. But it is magnified in a very tight turn...and the IS has a decent turning radius. Hope this helps.
I feel this a lot too and I'm glad that someone brought it up because I always thought that the wheels were being turned at such a sharp angle that I was riding on my rims :o ...but I've never thought of it that way CrunchySkippy, thanks.