As puppies are growing they require more food (as do we), this requirement reduces when they reach their adult life stage and therefore they are fed less.
If they continued to eat the same amount of food required for growth as they did through to adult life they would be over fed and put on weight.
Puppy feeding guides are based on the expected adult weight of a puppy in order to provide a more accurate feeding recommendation.
If the feeding recommendation was based purely on the current puppy weight – the breed size, age and growth phase would be unknown, making estimations of the feeding requirement unreliable.
People often are confused by the expected adult weight ranges – if a puppy will grow to be approximately the weight between two weight ranges (e.g. Puppy will grow to be approx 10 kg and the weight ranges are: 5-10 kg, 10-20 kg, how much should I feed?) The age ranges should be viewed as the average of the weights within that range (For example, 5-10 kg could be thought of as 7.5 kg).
The feeding level is enough to provide optimal nutrition for a young dog, without supplying excess nutrition to cause weight gain or cause young dogs to be hyperactive.