Ntile impala not dividing groups of even size

Question

I am using ntile function in impala to divide group of customers in equal size but it is not dividing in equal size. I am using below code :

`Table 1:

ColA, ColB, CustNo, Spend `

Data in table 1 is prepared at ColA, ColB, CustNo level.

Number of records in tabl1 is : 68555

My Code :

create table Table2 as 
  Select distinct * from (
    Select *,
    ntile(10) over (partition by ColA,ColB 
               order by Spend asc) as rank_10 
    from Table1
  ) a
;

When I am actually checking records for each decile I am not getting same records :

8|11931
6|4541
9|15048
7|9012
3|3118
4|3024
1|4635
5|2108
2|2847
10|12288

Please suggest the mistake