It's just the manufacturers cutting corners . . . 

3 votes for the funniest post of the year.

There are differences from manufacturers who had jobs farmed out to them...

http://www.flippers.be/pinball_playfields.html

As far as doors go, I believe there was 3 slots for the export market and 2 slots for USA machines.

JohnWartJnr may be able to answer it as he knows a fair bit about that stuff.