Indeed, it should be the same as the net diff.
i experimentet with a C code like this:
unsigned long x = 0;
x--;
printf("%lu\n", x);
unsigned long is 64bit, so when it overflows it becomes ~18.4E
therefore, what i belive - i might be wrong - is that cgminer actually gets an -1 error code when it tries to get the diff, and then it becomes 18.4E
or i might be totally lost..
but it does show a right value when im mining in a pool..