I noticed the tail of one of the chins became richer in fur. What could be the reason?

When I got the two from a pet warehouse, one was overall thicker and had a fluffy tail like most chinchillas and the other was more slender and his tail had almost no fluffy hair at all. No bite marks, just very short hair.He was also much more timid and more afraid of noises.

They were 1 year old. 4 months later I suddenly noticed that his tail became as fluffy as the other chinchilla, if not more.

Is it because of better nutrition and less stress? I switched to oxbow 1.5 month ago...

I noticed the tail of one of the chins became richer in fur. What could be the reason?