Why is the extra allowance for wheel installation on A380 in terms of degrees instead of torque/torque percentage? When installing a wheel on the A380, the nut is permitted to be torqued an extra X degrees if the holes do not align. 

Any idea why its in degrees instead of X torque or percentage of torque value?
The reason is simple, the measurement from the point correct torque is achieved to the location of the hole for the bolt is in degrees, not ft-lbs (the number of degrees is absolute, whereas the force needed to get the alignment can vary). The torque specification has a built-in tolerance for additional torque needed for alignment. If you have to go more than X degrees to align it means you'll be applying excessive torque.