I’m taking a graduate class in psychology tests and measures right now. If I find any information about your question I’ll post it here. From what I know so far, it is normal for retest answers to fluctuate. Being a self-reported test, the changes over time may represent changes in your knowledge or opinions over time.

The way I know for sure about having DID is through the eyes of my best friend, my therapist (and a couple other people), and the information I’ve recorded in my journal entries over the years. Also, memories and, well, my parts talk to me.
Yes, I guess a different part could take the test each time...but, if you knew that happened, that would kinda prove it right there.
One of my parts wrote in our journal about my denial, and how sometimes I think I made them up—he said it’s amazing how much credit I give to my will, as though I’m even capable of orchestrating all this stuff happening in this brain and life just on a whim. His comment really stuck with me.