Did you actually measured the new belt or just going by what the seller claims it is?
I personally know that Sunbelt and A&I belts are not sized correctly many times. Usually nearly 2" short on 5/8 belts. I through out over $1000 in belt last June because of this. The problem had got so bad that I quit buying belts from them but do rarely buy one, The two I got from A&I both were an inch short and was unusable.
It is the very reason I spent $200 on a v-belt measurer.