Perhaps we should clarify the situation, a little more.
As German Star states, the extra width does not matter, so long as you are going up in size.
However you will have wear problems, if you try it in reverse, as the belt will ride too low in the pulleys, and will quickly cause the re enforcement strip to wear, and next thing you will have a broken belt.
As a general rule also, you should not use imperial size belts, to replace metric belts, as often the width is too far away from your real standard.
When the belt is correctly tensioned in pulley, the out side of the belt, which is where the size marking are, should sit slightly proud of the top of the pulley.