Because when she shaves everyday she doesn't see the natural progression of the change in hair color. when she has to allow it to grow it is a surprise to see it all. Just like when someone starts coloring their hair, the do not realize the natural color is progressively lightening. I have had clients say "I have no idea what my hair color is any more."
Retorical Question: why are they white? why am I aging?.
I have a pat answer for this but it is soooo me and people laugh and love it--I won't share.
But I would say
"Such is the natural progression of everyone" with out pointing out she is aging because she is in denial.
Tweeze.--Try using more strips in more directions and pressing it on a little firmer