It helps to think of the sign as a direction. Above zero/below zero, credit/debit, above sea level/below sea level, etc. When the sign changes, you change direction.

So, +1 is "take a step," and -1 is "turn around and take a step" which means you move backwards one step.

With --1, you're essentially saying "turn around, then turn around, then take a step," which results in you just spinning about and moving forward just like with +1.