I don't know your circumstances and I'm not a professional, so my guess is merely conjecture... but I'm guessing maybe it's because you have different colors of hair?

Like you have some hairs that are darker and will respond well to one laser, and some that are lighter and need the other?
That was the case with me: I started going white at 16 and didn't start laser until 32, so half of my chin and neck were white. Not gray, not silvery, WHITE white. To complicate things, I also have every possible color of hair. On top of my head, it's roughly 40% brown, 40% red, 10% black (these are VERY thick and really curly), 5% blonde and 5% white. I could never grow a full beard, but what hair I had was roughly the same proportion as what was on the top of my head. The dark-hair laser could only get about half of what was on my face.
But after two sessions with the light-hair laser and seeing no results, we quit and went with electrolysis.