I came across this on the web. It's supposed to be funny, but we all know better :)
So, why are the numpad layouts different, and what are the reasons behind each?
There's this humongous article called Keyboard Trivia that has collected many of the theories and stories. The summary of facts:
There's also a theory that phone engineers wanted to slow down people who were fast at entering numerical data, which would jam lines and produce dialing errors, so they reversed the layout. However, records of AT&T Labs' research clearly invalidate it.
One additional possible explanation is that with the calculator layout many calculations can be performed along the bottom two rows and the plus button only; zeroes, decimal points and the enter/equals button.
Since the main buttons are all at the bottom, it makes reasonable sense that the other numbers would count up from the bottom too.
I suspect (but have no proof) that the phone keypad layout would make typing on a calculator more difficult for most simple calculations.