Its not like hair is in blocks of colour.
It grades and more usually is lighter at the front than at the back.
Wouldn't you want you top hair to do the same so it matches the lighter temple hair and gets darker towards the back?
You may be only replacing the hair on the top, but it has the meet the side and back hair at some point and you'd want the colour to blend in those areas.

Its just that I've seen it hundreds of time where people get their system and its too dark because they only sent a single sample.
Then you've got to work out if you're going to darken your natural hair or try and lighten the system and all you're doing is opening a can of worms for the sake of sending a bit more hair.