Maybe it was because having multiple negatives allowed them to create multiple prints simultaneously, saving time. Perhaps they only used the secondary negatives when pressed for time, making those more rare?
It might also have been done for variety (like with the N172's). Today we look for consistency in everything, but the producers of late 1880's mounted albumen advertising pieces weren't even very consistent with their inscriptions or mounts, much less photographic images.
__________________
$co++ Forre$+
|