That's just playfield numbers. They make a lot more play fields than actual machines. Hundreds more, in fact. The way that Stern serials games, makes it hard to track production numbers. This is mainly because at any given time, they are producing multiple titles and SOMETIMES, they just pull those serial number stickers from the same roll(s). But, it is not impossible to track, as some on Pinside have implied.
Here are some Stern hints:
They only use a serial # once.
The earliest (lowest serials) form each run are usually exported machines. This typically accounts for 40-60% of the run.
They rarely skip serial numbers, picking up right where they left off on the previous run of machines.
The lowest run is typically a block of 200 machines. (Not always, as some are 100. Some are even less..)
Multiple runs is the standard rule on machines.
The customers do not directly or exclusively determine the demand for a game, the distributors do.
There are always at least 2-3 titles being made at the same time.
The highest play field serials are always much higher than the number of games produced.
If you have a severely damaged or warped play field and send (or deliver) the defective one to Stern, they may swap it out for you if you can provide evidence, and are polite.
http://www.ipsnd.net/ is a great resource for entering and tracking serial #'s.
Sega/DE pins followed these same rules, for the most part.