The reason for that is that it did arithmetic via table lookup. As I heard from someone who was there, the University of British Columbia wanted to show off its brand new 1620, so they wrote a demo program, and with a host of onlookers, they pressed Memory Clear, then loaded the program, and it failed. Memory clear had erased the addition and multiplication tables.

LOL. I came here to make the same comment. I think the deal was they used a lookup table for addition instead of a modern ALU.