Two individuals start training at the same time, but one is able to grow muscle faster and larger than the other. what is the least likely explanation for this?

Every human has a different bodies with different kinds of chromosomes that build its structure. Chromosomes are inherited from their parents. Some people have stronger and more developed chromosomes which were inherited which than build its muscle faster and stronger. Some people don't have these chromosomes that are developed and they will need more time to develop their muscle.

True. the more you know, the better of you will be.