1671E

Why the answer is 16 but not 8 in Example 1? I can only figure out the 8 strings below

1. B AAAAAAA ABBABAB
2. B AAAAAAA ABBABBA
3. B AAAAAAA ABABBAB
4. B AAAAAAA ABABBBA
5. B ABBABAB AAAAAAA
6. B ABBABBA AAAAAAA
7. B ABABBAB AAAAAAA
8. B ABABBBA AAAAAAA