Hey Grayman, could you add more to your comments. It is not clear on which list you are looking at, Pinside? IPDB? or other.

So I entered "1960 to 1965 all pins" The listing that shows up has 1965 at the top. ascending to the 1960 games. Only a page at a time, not in rows.

At the bottom of the page, "Pinside Game Archive", there is an option for ascending or descending, Is this the situation?

I have duplicated the same glitch as claimed by OP
when one scans the games list from present (2017)
and when going back thru each new page as it goes back in time
when you get to page 18 you have games from 1965 with 4 games from 1964 at bottom of that page
but when you go to next page (#19), it jumps to 1963.
Obviously there are more games from 1964 that this pinside games list is not listing.
When i narrow the search for pins from 1963 to 1965 and paginate thru the list
all games from those eras (years) are listed, with out any missing from 1964.

I posted glitch in the bug section forum
and Robin acknowledged the problem
and thinks he found the cause,
but may take some time to fix.