sqlite sum, strftime, group by

I have this table for examples:

id  cp  base        qta  data_in
1   0   BaseBianca  30  2015-02-01 13:09:55
2   0   BaseBianca  32  2015-01-10 13:38:58
3   0   BaseBianca  45  2015-01-05 13:55:09
4   0   BaseBianca  34  2015-01-15 13:55:19
5   0   BaseBianca  46  2015-01-01 13:56:53
6   0   BaseGialla  45  2015-01-05 14:17:05
7   0   BaseNera    33  2015-02-06 14:17:34

and i would like to obtain the total qty of base in every month. this is my sqlite query:

select base, strftime('%m', data_in), sum(qta) as total from tb_basi group by base 

but this is the return:

base        month  qta
BaseBianca  01     187
BaseGialla  01     45
BaseNera    02     33

but the correct return table are

base        month  qta
BaseBianca  01     157  
BaseGialla  01     45
BaseNera    02     33
BaseBianca  02     30

how can modify the query?