Quoted from goatdan:

Contrary to popular belief, this simply wasn't the case. If you were getting pins back in the day, it was amazing how many came off route with old, horrible code on it that was worse than what Stern games ship with today in general. NGG has a software bug in it to this day. DESW (and yeah, I know, Data East is Stern...) had never got some stuff implemented until just recently that was in the original ROM. When I got my DM originally, the ROMs in it were so old that it had only a skeleton of a ruleset... and it had been in someone's basement like that for about 10 years (it was HORRIBLE).

And back then, there was really no chance to even update your pins code easily. It was an expensive undertaking.

Besides that, the runs were much longer, so an early DM with bad code on it may have only shipped on the first say 200 machines before they updated it for the next 9000 (or whatever).

Now, when collectors trade the games, it looks like all of the B/W games are fully updated and must have been like that forever. It was with some, but definitely not with all. Which seems to be exactly where we are today, where some games ship basically complete, and others need overhauls.