By the very definition of transpose(say T) of a matrix M, we have M[i, j] = T[j, i]. But here since we are merging matrix A with transpose of B, the diagonal elements of A and B will have to be preserved in C, therefore,
A[i, j] = C[j, i]
B[i, j] = C[j, i+1]
Here B[i, j] cannot be equal to C[j, i] because presence of elements of A at the diagonal of C shifts the elements of B by 1 column.
I would like to know a formal explanation though.