[Pat R]

Just speculating if some sheets could have had the fronts printed and then
been cut down before the backs were printed.

For example say a sheet with 8 verticals of the same subject with 4 verticals
of another subject on top had the fronts printed, the sheet is then cut 2/3 X 1/3 horizontally and the backs are printed on some of their smaller printers
with the bigger 2/3 sheet having the more popular brands and the smaller
1/3 having the less popular brands printed.

This would utilize the different size printers they had and account
for the small number of no prints in the popular brands and the larger no prints in the less popular brands.

The wording on the right is from the middle enlarged so it would be readable.