Right you are! I'm not playing the game, but based on what you shared, that equation should equal 13.
Operations in paratheses first, then operations on multiplication and division, from left to right and finally addition and subtraction from left to right.
The "addition before subtraction" rule is often taught to children in elementary school. However, some teachers prefer the reverse. When we move on to algebra and other "higher" mathematics, we're taught how to handle more complex equations. Thusly: Ask Dr. Math!
However, next time you play that bit, is negative 5 an option? Might be the developers used an obscure rule most real mathematicians don't use.