What I'm trying to figure out is, from a design standpoint, how do they calculate this rating.
I'm not sure anyone answered this directly, although the answer may have become obvious through the course of the conversation.
A designer calculates the actual DEMAND load that needs to be supplied by the transformer. What you are doing is looking at the CONNECTED load, which is completely different. One the designer knows how much current is actually needed, they use that number to size the transformer.
For example, you used the number 225 A to get 81 kVA. In reality, that 225 A may only be using 100 A, in which case the transformer does not need to be as large as 81 kVA. A 50 kVA would be plenty. Yes, it is a good idea to allow some room for expansion, but as iwire has already pointed out that costs money and tends to get cut out of the project later when the budget gets tight.