I couldn't tell you how it got there, if I could I'd be either the prosecution or defence star witness
I have cut myself in my car in the before (long story!) but it's not necessary for the cut to have been inflicted by something in the car. She could have cut herself on anything outside the car and transferred it to the interior, it could even have been a sharp edge on the metal immediately adjacent to to the stain. It's in an area where it's not at all unlikely that she would have come into contact while assisting a child in the back seat for instance (other possible reasons, but you get where I am going I am sure).
We will know in the coming days whether the jury has an issue with it. Personally I do have an issue with it, but 12 other people are perfectly capable of coming to an entirely different conclusion and well, let's hope so for the sake of Allison and her family. While I try to main completely objective, a large part of my working life (as a non verified expert of course) has been dedicated to exploring alternate, and sometimes even unlikely scenarios so I hope you can understand my ingrained perspective may alter from the majority at times (and again, let's hope this is the case).