why a STORE_FAST after INPLACE_ADD

python 2.7

In [20]: def foo():
....: a += 1
....:

In [21]: dis.dis(foo)
2 0 LOAD_FAST 0 (a)
3 LOAD_CONST 1 (1)
6 INPLACE_ADD
7 STORE_FAST 0 (a)
10 LOAD_CONST 0 (None)
13 RETURN_VALUE3