Originally Posted by Andrew Yiu

No. There are a few different variants of 763 in the fleet; those are the only two 'types' that have been/in the process of being XM so far, that's why only those 2 types are under the XM seat maps and total doesn't add up to total fleet as the rest of the planes are different types. (31)
If you're referring to those 2 specific types - one layout has no row 17 so that's one way of distinguishing it.