Paul, I have noticed the same thing....I have no explanation for why some of the lowered numbered cards (with lighter borders, #s 1-64) are on the same sheet as cards with the darker bordered cards. I have not seen an uncut sheet to see the layout Bowman used. However, this old thread at least mentions how the sheets were laid out (8 across x 4 rows ).
http://www.net54baseball.com/showthread.php?t=120822
It would make sense that if there are 32 card sheets, that there should be two full sheets for the first 64 lighter bordered cards with no darker border cards included.
In flipping through some of my cards, I found card #s 25-30 all have darker border cards to their left edge, and found cards 34, 35 and 40 have darker border cards to the right.