I am not going to tell you what I think this could be but I will walk you through the facts as you wrote them.
You can gain two tissues while eating at a caloric surplus.
One can be flexed and the other can not be flexed.
Yeah you can get bigger overall and not be able to flex the new tissue. See above^
One of the two tissues I mentioned above is denser than the other.
This would give the illusion of becoming bigger while losing weight.
So without giving my opinion, which tissue do you think you gained since it would explain all of your observations?
Tissue Most Likely Responsible:
A.[ ] Muscle
B.[ ] Fat
C.[ ] Water
D.[ ] A walnut
E.[ ] Both A & B
F.[ ] None of the above