"Corremos en el parque todos los sábados."
Translation:We run in the park every Saturday.
Because it's not exactly the same thing, I'd imagine. It's close enough that I probably would have expected it to work, but personally I don't use "every Saturday" and "each Saturday" quite interchangeably.
Because, programmers got lazy and didnt want to put every single input possible as correct. Just report it and move on