WHERE clause is ignored with UNION

tpch> select c_custkey, ret
> from (select c_custkey, ROUND(sum(c_acctbal*15000000)/sum(15000000),4) as ret from customer group by c_custkey
> union
> select c_custkey, ROUND(sum(c_acctbal*15000000)/sum(15000000),4) as ret from customer group by c_custkey) a
> where ret > 0.02;
c_custkey,  ret
-------------------------------
64568,  3631.24
26627,  9070.85
105296,  2863.05
51704,  7404.84
111424,  -977.6
32824,  3718.26
138521,  -510.01
...