2001 BMW 530 Weird symptoms!

Hi, great site and very informative.
My BM' has good brakes (ABS too), but during a recent safety inspection, it was discovered that the inner surfaces of both of the front discs (rotors) were in poor shape, i.E: going rusty with very limited pad contact. It looks like there's no braking on the inner sides of both front discs, although the outer sides are in very good condition and as one would expect to see from good pad contact. The calipers are single piston / floating jobs, so I can't understand why the outer brake pads seems to be operating okay, when the inner ones don't! Surely any seizing (or 'hanging') by a stiff piston or stiff caliper slides would affect both pads of the defective caliper? The car doesn't do much of a mileage, as you will see from it's age, but this one's really got me stumped; any ideas?
Cheers,

Wullie.