Strange! Also strange:
rng(1) ; A = randn(100,3) ;
B1 = A'*A
B2 = mtimes(A',A)
disp(['B1 == B2 : ' '0'+isequal(B2, B1)]) ; % ?????
% but
X = A' ;
C1 = X*A
C2 = mtimes(X,A)
disp(['C1 == C2 : ' '0'+isequal(C1, C2)]) ; % YES!
disp(['C1,C2 == B2 : ' '0'+isequal(C1, C2, B2)]) ;
I first thought that your C is created using MTIMES:
C = A(:,:)'*A(:,:)
disp(['C == B2 : ' '0'+isequal(C, B2)]) ; % but no
disp(['C == B1 : ' '0'+isequal(C, B1)]) ; % also not!!
So there are at least three different results, that should be the same ...
Very puzzling!
