Why do two dogs of the same breed have different healthy weights?

Breed standards typically list a weight range, sometimes quite wide, precisely because individual variation within a breed is normal and expected, similar to how human height varies within any population.

A dog at the lower end of its breed's typical range is not automatically underweight, and one at the upper end is not automatically overweight, provided body condition looks appropriate for that individual frame.

Frame size, meaning the size of the skeleton itself, is the real determinant of a dog's individual healthy weight, and it varies between dogs of the same breed due to genetics, just as it does in people.

A smaller framed dog carrying the breed's average weight might actually be overweight for its own frame, while a larger framed dog at the same weight might be perfectly lean.

Use body condition scoring, checking ribs, waist, and belly tuck, rather than comparing your dog's weight number directly to another dog of the same breed, even a littermate.

If you are unsure what frame size your dog has, your vet can assess this alongside body condition to give you a more personalized sense of an appropriate target weight. This is especially useful for a mixed breed dog, where no single breed standard applies and a hands on assessment is the most reliable way to judge what is right for that individual.