Could just be an animation error. Those things tend to pop up from time to time in SU.

Maybe it was the result of shapeshifting, or she can alter her fingers to resemble a "normal looking" hand. Either way I think would be a viable explanation. Meanwhile I'm just wondering when she'll show up again and if the popular headcanon that she joins the Gems at Steven's request (under the basis she's "not really evil" but misguided due to just following orders) becomes true. (yeah watch this comment become pretty outdated soon enough, heh)