Its random. The base defense will reroll. So even if perfect ED the base defense will vary when u up it or double up.

The uograded défense is already written in the item too. If you dupe the item and upgrade both the same item. They will have same défense After upgrade . I tested it .

you cant test this proposal, i can always be coincidence. you could only test the other way round
how large was your sample size?