Since all 8 sprites are used and also 8 bitplanes, there were fewer DMA slots available for the blitter and the only way to avoid slowdowns was to use 32-bit fetch mode for the bitplane data (and thus fine scroll by 32 pixels instead of 16). So these 16 more pixels to cover with border are exactly the difference between the 304 pixel wide screens on OCS/ECS and the 288 pixel wide screens on CD32.
Posted by ross on EAB