Im not going to comment on the "rarity" of it, but it must have something to do with the separation of the "P". One of my friends got Elvis after a concert in Vegas in the 70's and she said he was so high he could barely scrawl his name. so I am not surprised to see variations, especially in his later signatures. While the signature itself is not what you would call "vintage Elvis" the handwriting looks spot on, and there are enough positives in the signature where I would lean towards authentic.
Don, enjoy these Elvis brain teasers! Some day I will have a good idea what a genuine Elvis looks like or be more confused as ever. Don't know what would make it rare but I'm thinking this is the real deal. Compared to the huge amount of fakes around I would say what makes it rare is that it's authentic. :0)