Methinks it refers to how the printer cuts up a sheet of paper. 8 graphic novel pages might be printed on one big sheet, which would then be cut up and sorted into page order, to be assembled into the book. You can't have blank pages so either the graphic novel's length is 32 or 64 or 128 pages, etc. (or any multiple of 8) so the print run is x number of printer sheets each containing 8 pages, or else you fill any remaining blank sheets with other content.
That's not to say you won't find printers who, using smaller scale equipment, can print your graphic novel directly onto single sheets. I see comic makers doing this all the time. Depends how thick the graphic novel is and whether it can be bound by the smaller scale equipment.
-Derek