Brake Not rubbing rust off disc?

Another possibility is the pads that have been used may be chamfered on the edge.

It's done to reduce brake squeal/noise. As the pads wear, the chamfered edge will come into contact with the disc and the full pad area will be used.

as said by jags,sometimes this happens with new pads as they have schamfered edges on the pads,the disks couldalso be lipped and not allowing the pads to fully contact with the disk,altho as many have said it could be a case of the wrong pads being fitted.