Very dark hair like Reubin's tend to have dark knots where they are tied to the lace base and can be obvious along the front hairline.
Like little black dots.
When hair is this dark, its not always possible to bleach the knots so that those dots disappear.
So Reubin, and others, order hair that is blonde at the front and the dye the hair darker to match the rest of the piece, but avoid the knots.
That way they have bleached knots you can't see, but dark hair all over.

I hope that helps.