For instance, if it is against the law to drive over the white line on the street corner, we understand that the line has to be drawn in a particular spot, but would be amazed if a policeman would cite us for going over the line by a mere 5 cm. Legally though, one side of that line is permitted, and one side is prohibited. In halakha, exact definitions are as binding as the laws themselves. If one is commanded to eat matza on Pesach, then one has to know exactly how much is the minimum needed to fulfill the obligation. In real life, it is often impossible to achieve such exact definitions.
Life has rough edges.
The law is ideal, and attempts to impart that ideal clarity to imperfect existence. This too reflects the divine origin of the law. The law represents the ideal, not as an unattainable source of inspiration, but as the mission of man to impress God's will on reality, to bring the imperfect world up to perfect standards. Each individual act is made to conform to exact standards, and in so doing, brings the soul, the inner infinite depth of man, closer to God, to its ideal image and potential. We can control the definition of an action, and so impart to our inner selves the pull of infinity.