Jo's right! If it were 6/8 & you took that to the lowest denominator, it would be 3/4 - so 5/8 is less than 6/8 & 6/8 would be equal to 3/4! Does that makes sense!
You don't need to a master saddler to know that 3/4 of a inch is wider than 5/8ths of an inch! It is the same whatever you are measuring. All you need is a (very) basic grasp of simple maths.