Why are daily values only a general guide you need each day?

Because daily values are calculated according to the needs of an average person.

Meaning they'll be about right for every one but maybe not perfectly right for everyone.

If you're above average height/weight/activity level/muscle-to-fat-ratio etc etc you might need more of this or that.

If you're below average average height/weight/activity level/muscle-to-fat-ratio etc etc you might need less of this or that.

You might have a medical condition or an ethnic origin that makes you need more of something or unable to process another efficiently.

People of African or Asian ancestry might benefit from more vitamin D for instance if they live far from the equator or spend much time Indoors.