It's a good question. A fixed phrase perhaps? Does anybody know?

When you count things, you count a row of things; like counting sheep to fall asleep, or counting apples to see how many you have. But when you calculate something you use an abstract process of reasoning, weighing things up against each other, using formulas etc.

"We calculate it", or "We work it out" is the usual English expression.

"We figure it out" is yet another synonym, but Den Dansk Ordbog mentions only the Danish translation, and not the English/American.