A careful maker knows which measurements matter only after someone sits down.

Drawings do not sit down. A maker can get every dimension right on paper and still find the height wrong for the way people actually settle, the arm in the wrong place, the back uncomfortable after an hour. Certain measurements only become important once a real body meets the object, and no amount of care beforehand produces that information. Testing is not a shortfall in skill, it is where the last of the skill lives.

Put whatever you are making in front of one real user early. Watch what they do rather than asking whether they like it.