Paul, I have a couple of thoughts on this one,
while attempting to remove the stain the customer inadvertly removes the protector allowing the stain to penetrate deeper.
a stain might have a high viscocity so will be too 'thick' to penetrate deap into the fibre, by diluting the stain with a spotter the customers lowers the viscocity, so allowing the stain to penetrate deeper.
i have no scientific proof for any of these theorys, but the sound plausable
Mike