A companion on the same PNR as you will have the same upgrade priority as you. If you check-in together, you will only upgraded together. However, if you check-in separately, you can be upgraded independently. I suspect that your companion likely checked-in prior to you and therefore had a higher upgrade priority (rather than it being alphabetical). At any rate, your example appears to have very little to do with OP's case.
Op said he booked 13 hours later which means different pnr. They are not companions. They just have the same itenerary and same fare class. We can either assume 1- deltas IT backend is so complicated they don’t even know how to fix it. Or 2, they know exactly what they are doing and have a system for going out of order to keep platinums happy. Or 3, op got the facts wrong. I believe option 2.