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Well that could be his signature pose too. Forgot about that and when he also has one hand behind his back while holding his keyblade with the other.I always thought his signature pose was the one with his hands together behind his back.
I had a feeling it was like that when I first saw the cover that Xehanort was on top and everyone else was on the bottom. That is very interesting.It's kind of his look in promotional media.
In the Birth by Sleep CGI artwork, you can see that his fist is raised, almost into a ball, representing that everything is "almost in his grasp". It also toys with the expression "I have them in the palm of my hands".
This is replicated in the Kingdom Hearts 2.5 ReMIX box art where his hands are outstretched, giving him the menacing feel that everything in the series isn't fate or destiny, but rather had been planned by him. This is evident by the protagonists being within the field of his arms.
He's not trying to grab the keyblades, but rather Nomura is trying to show that, in this collection, Xehanort is the puppet master behind everything, and these protagonists are the puppets.