PJD1, I don't think you understand the concept of what a Volume licence means in this situation. It refers to the sort of agreement that a company has with Microsoft whereby the volume of purchases they make entitles them to various discounts. It doesn't refer to the number of licences of a particular product that have been acquired.
It is perfectly possible for a company to have bought several hundreds or thousands of licences under a volume agreement and to subsequently sell them on following a corporate upgrade to a later version, where the company acquires new licences for the later version, and the original licences are no longer in use.
Of course, this situation tends to be uncommon, as organisations tend to only upgrade when they have to, so the old product has little or no usable support life left and therefore has negligible resale value, or else they pay annually for software updates, where the update subscription allows the perpetual licence holder to upgrade to the most recent version of the product without having to buy new perpetual licences.
FBW