If you’re typing on a full size keyboard there’s a good chance that to your right, there’s a number pad.
The number 5 is in the middle – That’s to be expected, but is 1 in the top left or bottom left?
Being derived from an adding machine keypad, the number pad on a keyboard has a 1 will be in the bottom left, however in the 1950s when telephone keypads were being introduced, only folks who worked in accounting had adding machines.
So when it came time to work out the best layout, the result we have today was a determined through a stack of research and testing by Human Factors Engineering Department of Bell Labs who studied the most efficient layout of keys, and tested focus groups to find the layout that provided the best level of speed and accuracy.
That landed with the 1 in the top left, and that’s what we still have today.
Oddly ATM and Card terminals opted to use the telephone layout, rather than the adding machine layout, while number pads use the adding machine layout.
A few exceptions to this exist, for example the Telecom ComputerPhone (Aka the Merlin Tonto in the UK, or the New Zealand Post Office Computerphone, or the ICL One Per Desk) which is the keyboard as envisioned by the telephone company.