"Rechnen" is short for the now obsolete "rechenen", which seems to be where the words "Rechenzentrum" and "Rechenaufgabe" came from. It's a word that predates what we call computers today, too.

A "Rechenzentrum" was a place where lots and lots of people were sitting down crunching numbers with pen on paper. This job also existed in the USA, where originally "computer" was the job title of someone working in such a computer center.

But then electronic computers became better at crunching numbers than human computers, so the job became obsolete. The word stayed around, but the meaning changed. Same way we dial phone numbers despite phones no longer having dials. We also still hang up the phone, despite there being nothing to hang the phone on anymore.