In LCS problem, Why can’t (len[i][j]=len[i-1][j-1]+1;) just be (len[i][j]=len[i-1][j]+1; )?[Dynamic programming]

Question

 for (int i=1; i<=n1; i++)
        for (int j=1; j<=n2; j++)
            if (s1[i] == s2[j])
                length[i][j] = length[i-1][j-1] + 1;
            else