As you feed across the work, the theory goes that, the leading edge of the wheel wears (very slightly), but the remainder of the width of the wheel remains at the diameter to which it was dressed so the ground surface becomes flat (or as flat as the grinder!)
Remember that feeds are exceedingly small on finish passes, measured in tenths. I was fairly gulping that plate in 2 thou bites as this isn't the finished thickness of the part yet - I just wanted it flat enough to get a bore true to the surface.
I started turning the plate into a true disk on the manual lathe. Firstly pressing the arbor firmly home using the arbor press, then placing of a 'soft centre' turned in place in the 3 jaw to avoid changing chucks etc.
This wasn't satisfactory, as the intermittent cut was pounding the soft centre and slowly putting it out of true.
So I mounted up a hard centre in the spindle bushing, and used the catch plate. A process rather delayed by seeking the catch plate -WHY was it in the cupboard with all the Bridgeport bits, NOT the cupboard for the Colchester bits

Oh well it only took an hour to find it !
Haven't finished turning it yet as I've been called away for canine supervisory duties
