Python's random algorithm uses a seed. It's not truly random. You won't get a different seed by hitting the back button. Reloading the game might change the seed. I know Ctrl + R does when I'm coding out of the command console.
The atk score is calculated by talking 10 + 2d6("random") + weapon bonus vs the opponent defense score. You might have been going against someone with stupid high (17+) defense.