Those quite often follow Benford's Law . That means if you are using scientific notation, where numbers consist of one digit to the left of the decimal point, the decimal point, and then a fractional part, then an exponent for the power of 10 the number is to be multiplied by, then you are much more likely to be entering a small digit before the decimal point than a large digit.
123, therefore, arguably belongs near the decimal point on the keypad on calculators.
The numeric keypad is largely based off of ribbon tape desk calculators , which is why the + is double size, the enter (=) is double size, and the 0 is double size. The + and Enter are designed to be used by the right hand pinky, while the 0 is designed for use with the thumb. The numeric keypad is designed for five finger entry. Leaving it in that configuration means that people in an office who are used to using a ribbon calculator can more easily adapt to the PC keyboard. That's what office workers expect to find on a machine that sometimes is just a calculator.
If you go into a business or accounting office today, you'll still see people using these stupid calculators (stupid because they waste paper and ink). And if you've ever seen someone use one that's good with them, they can be extremely fast. Faster than you'd ever need to be entering a phone number (unless you need to enter the IT Crowd's emergency number).
When you use a telephone, however, you only use one finger. You're only entering 7 to 11 digits, typically.
Everyone or nearly everyone in the user study was surely a telephone user. At the time, that meant being accustomed to the rotary dial layout, where the order of the digits is 1234567890. 0 is the last digit, after 9.
Both the calculator and touch tone layouts had 0 at the bottom. So the calculator layout has 0 next to 123. The touch tone layout has 0 next to 789 (and following the 9), just like a rotary dial.
This must have accounted for some of the preference between these two layouts.
I have yet to see the numeric keypad on the left for left handed users that type in lots of numbers, e.g accountants, data entry clerks etc. We also never had desktop computers integrated into landline phones with a 'phone lock' button to switch over to 'dial someone' mode. At all times the numeric keypad has opted to follow convention as defined by the original IBM PC circa 1981.
Multi-column calculators survived much longer, being king of the hill for high-accuracy multiplication and division. I had a job using one until the end of 1969.