Union issue in SAS

Question

I have met an issue about union in SAS.

1) When I run the code below:

proc sql;
select count(*) as cnt_obs, count(distinct user) as cnt_user from input1
union all
select count(*) as cnt_obs, count(distinct user) as cnt_user from input2
union all
select count(*) as cnt_obs, count(distinct user) as cnt_user from input3
union all
select count(*) as cnt_obs, count(distinct user) as cnt_user from input4
quit;

I got the result:

cnt_obs    cnt_user
10000000   4983437
9771110    4983438
8345177    4983438
8188069    4983438

2) But according to my process prior to this proc sql step, the second column of the result should be the same, so I run another code to check my result. When I run the code below:

proc sql;
select count(*) as cnt_obs, count(distinct user) as cnt_user from input1;
select count(*) as cnt_obs, count(distinct user) as cnt_user from input2;
select count(*) as cnt_obs, count(distinct user) as cnt_user from input3;
select count(*) as cnt_obs, count(distinct user) as cnt_user from input4;
quit;

I got the expected result:

cnt_obs    cnt_user
10000000   4983437
9771110    4983437
8345177    4983437
8188069    4983437

3) It seems there's an issue with UNION in SAS proc sql step, does anyone know what the problem is? Thanks very much!