1729 is actually his chassis number.

Bender gets an xmas card from the machine who made him that says "merry xmas son 1729" It's not his serial number but is sort of close to a serial number in a way.

http://en.wikipedia.org/wiki/1729_(number)

This is really weird, because the article quotes a mathematician/writer for show: "For example, I chose Bender's serial number to be 1,729." The article talks about how 1729 is special because 1729 = 1³ + 12³ = 9³ + 10³ (and is apparently the smallest number that is the sum of two different sets of cubes or something like that), but the wikia article says 2716057 = 952³ + (-951)³. It's like they are similar ideas but the numbers are actually different. I guess the writer was misquoted, or forgot which sum of cubes he chose...

Could have just been an oversight, there's quite a lot of them in Futurama when you look closely. They often change history to match their own story at the time, like Leela's mother originally had normal arms rather than tentacles in the earlier episodes.