@Derahobs If the intention was to add the 99 and 1 first, it would need parentheses like this: 100 - (99 + 1) = 0.
Without the parentheses, going strictly left-to-right gives us 2!
@Derahobs Addition and subtraction have the same priority in the order of operations, you simply solve the equation from left to right:
First, you do 100 - 99, which gives you 1.
Then, you take that 1 and add the final 1, which gives you 2.