OK, so I think you're saying that each of the other two books was the 25-cent issue sandwiched between the last 15-cent and the first 20-cent issues?
He's talking about how the books were bound (or physically assembled). The majority of the 25¢ Picture Frame books have square (or flat) spines. While all the books were 52 pages, Captain America Annual #2 and My Love #14 were bound like a typical 36 page book from that era. They were stapled in the center of the spine (saddle-stitched).
The rest of the 25¢ Picture Frame books are square bound. They're not stapled in the center of the spine. The two staples go through the interior pages then the covers were glued onto the interiors. So if these books were properly assembled, the spines are flat (or square).
Good examples to explain the difference are Amazing Spider-Man Annuals. I believe ASM Annual #1 though #13 are all square bound. Then they switched to saddle stitched for #14 through #21.