WHY? UNION ALL is doing a Calculation

I have two data sets: DS_A and DS_B.

My question is why I have 2 quantity for pro_id 71549 and not 3?

The ALL Incorporates all rows into the results. This includes duplicates. If not specified, duplicate rows are removed.

loc_id        pro_id      quantity    price
------------- ----------- ----------- -----------
2310          5052        1           0
2365          5433        1           0
2310          7694        1           0
2310          9480        1           0
2310          9502        1           0
2310          14413       1           0
2310          31277       1           0
2310          46180       1           0
2310          65233       1           0
2310          68369       1           0
2310          68372       1           0
2310          77396       1           0


loc_id        pro_id      quantity    price
------------- ----------- ----------- -----------
2310          71549       3           0

When I do my UNION ALL

select loc_id
   , pro_id
   , sum(quantity)
   , price
from DS_A
group by loc_id, pro_id
UNION ALL
select 2310
   , 71549
   , sum(quantity)
   , price
from DS_B
group by pro_id, loc_id

Results:

loc_id        pro_id      quantity    price
------------- ----------- ----------- -----------
2310          5052        1           0
2365          5433        1           0
2310          7694        1           0
2310          9480        1           0
2310          9502        1           0
2310          14413       1           0
2310          31277       1           0
2310          46180       1           0
2310          65233       1           0
2310          68369       1           0
2310          68372       1           0
2310          77396       1           0
2310          71549       2           0