What's puzzling, really? Prior to WWII, the USA were hardly a "beacon of knowledge". They were still a young country and most scientific advancements originated from Europe.
[noun]-er as in [noun]-person, like butler, computer, farmer, etc. The device in front of you is _electronic_ computer as called in the era it had been written in.
As American printers were not yet accustomed with the practice of typesetting mathematical symbols, they needed to be taught.