Maybe bone density. Bone weight vary's by bodyweight. Powerlifters bones weigh more than a supermodels, because of just carrying excess weight.
Maybe Water hydration ?? Who knows!?
In my case, I made size improvements all the way around. I gained muscle. That was confirmed by calipers.
If you equating "gaining muscle" by the scale, then technically no. But if your equating gaining muscle by circumfernces vs. Body fat, I gained muscle. Around 7 I would assume.
Adding this later: Most of my 215 weight was in the gut, which i took from 38 inches to 31 inches, with my arms starting at blazing 13 inches flexed, to a blazing 14-1/4 flexed.
In my mind, I gained muscle, totally.