一个mysql不解的问题

执行语句1

select count(distinct uid) from usership ;

假设得到结果数据 10000

执行语句2

select count(distinct uid) from usership where groupid not in (select id from usergroup where fleet=1);

得到的

结果是 > 10000

select count(*) from (select distinct uid from usership where groupid not in (select id from usergroup where fleet=1)) t

得到的结果是 <10000 符合预期的