Interesting! I decided to do some detective work and find out why. I found this: http://imkevinxu.com/xkcd/parser.js, and thought that maybe if I knew how the strings were parsed, I could figure out why it was doing that.

    $ string_eval("x*x1")
    "x*x1"

I figured that the issue must be with the equation drawing or evaluated. Here's what I found as an inline script (note that i is points at which the function is evaluated):

    current_expression = expression.split("-x").join(-i);
    var result = eval(current_expression.split("x").join(i));

So it just splits up the equation and rejoins it. From here it was pretty easy to see what was happening:

    $ expression = string_eval("x*x1");
    "x*x1"
    $ expression.split("x").join(12);
    "12*121"
    $ expression.split("x").join(3);
    "3*31"
    $ expression.split("x").join(0.3);
    "0.3*0.31"
    $ expression.split("x").join(-3);
    "-3*-31"