I saw the signature & it looks legit! I have seen a lot of signatures over the years & I am no expert but I have noticed all of Dimes signatures are not consistent , but in my opinion the one you are getting is 100% real , cuz I can just tell the way he signs with his last few letters in his name as they all have a tendency to lean to the right. Dimes earlier signatures looked totally different , after CFH he started changing his signature , where he started making that cooler looking "D" in the beginning of Dimebag ,& still some look different from others. Trust me every time I met him and got signatures from him on the spot, well they all looked a little different & that's me getting them from the Man himself! Plus u never know if Dime was drunk when he signed certain ones, wouldn't surprise me! Bottom line I would not worry cuz I have a signature that looks just like the one on the guitar u just got.
Also on certain models like the Trendkill one he did not sign on the guitar because it was not a plain solid color guitar, so guitars like that with a lot of graphics, he would sign on the backplates, so that also let's me know it's REAL!