The secret rules of the game.
Commutative and Associative laws.
Euclid proved these properties in ancient Greece.
a + b = b + a. Always.
Plain English: Addition is friendly. It doesn't care what order you do it in. 5+2 is the same as 2+5. You can swap them around to make math easier.
Real-world example: Loading a truck. It doesn't matter if you load the big box then the small box, or vice versa. The total weight is the same.
Thinking this works for Subtraction. It does NOT! 5-2 is not 2-5.