The Print-on-Demand (POD) publishers have specific limits to the number of pages their presses can print on demand. Because all three volumes of the RE scriptures are too lengthy to print using most POD machines, each volume was split into two books, for a total of 6 individually printed and bound items.
The leather-bound scriptures will be custom-made by special order, and so they don't have the page limit restrictions inherent with POD printers. So, these special-order RE scriptures will retain the original 3-volume layout of The Old Covenants, The New Covenants, and Teachings and Commandments as originally envisioned by Joseph Smith, and ratified by the body of believers. A Glossary of Gospel Terms has also been added to the Teachings and Commandments (T&C) as an appendix.
Either way, when an entire set is purchased, both the POD and the special order versions include the same content, layout and formatting.