Checklist question
 

I have noticed this for years in 1960's checklists.

An example is the 1961 Topps 7th series checklist. The checklist begins with #507 Pete Burnside. High numbers are deemed to start with #523 Joe Gibbon, atleast as far as book value and when you start pricing "high numbers".

This is the case with checklists atleast through 1966, maybe further.

I know the obvious answer is that card #507 - #522 was issued with 6th series cards, but why did Topps do this? Is it the number of cards on the sheet and they did not want any DP's?