在外键上使用 order by 的左外连接返回带分页的重复项

我有两个表 api_user 和 api_user，其中 api_user 具有用户表的外键。这两个表的架构如下所列。

                                 Table "public.api_user"
   Column    |           Type           |                       Modifiers                       
--------------+--------------------------+-------------------------------------------------------
 id           | integer                  | not null default nextval('api_user_id_seq'::regclass)
 is_admin     | boolean                  | not null
 is_agent     | boolean                  | not null
 is_guide     | boolean                  | not null
Indexes:
    "api_user_pkey" PRIMARY KEY, btree (id)
Referenced by:
    TABLE "api_userprofile" CONSTRAINT "api_userprofile_user_id_5a1c1c92_fk_api_user_id" FOREIGN KEY (user_id) REFERENCES api_user(id) DEFERRABLE INITIALLY DEFERRED


                                         Table "public.api_userprofile"
         Column         |          Type           |                          Modifiers                           
------------------------+-------------------------+--------------------------------------------------------------
 id                     | integer                 | not null default nextval('api_userprofile_id_seq'::regclass)
 percent_complete       | numeric(3,0)            | not null
 display_name           | character varying(128)  | not null
 city                   | character varying(64)   | not null
 user_id                | integer                 | not null
Indexes:
    "api_userprofile_pkey" PRIMARY KEY, btree (id)
    "api_userprofile_user_id_key" UNIQUE CONSTRAINT, btree (user_id)
Foreign-key constraints:
    "api_userprofile_user_id_5a1c1c92_fk_api_user_id" FOREIGN KEY (user_id) REFERENCES api_user(id) DEFERRABLE INITIALLY DEFERRED

当我运行以下查询时：

select 
    api_user.id, 
    api_userprofile.display_name, 
    api_userprofile.city
FROM "api_user" 
LEFT OUTER JOIN "api_userprofile" ON ("api_user"."id" = "api_userprofile"."user_id") 
WHERE ((("api_user"."is_admin" = false 
    AND "api_userprofile"."percent_complete" >= 60.0 
    AND "api_userprofile"."id" IS NOT NULL)) 
    AND "api_user"."is_guide" = true)
ORDER BY "api_userprofile"."city" ASC LIMIT 20;

它返回：

id  |       display_name        |           city           
-----+---------------------------+--------------------------
 299 | Mohsin Khan               | Agra
  93 | Rizwan Mohd               | Agra
 126 | Abdhesh Sharma            | Agra
  39 | Rashid Ahmed              | Agra
 244 | Nishkam Sharma            | Ajmer
  42 | Parminder Mahla           | Amritsar
 131 | Prashant Hullatti         | Ballry
 241 | Pankaj Anand              | Bangalore
  89 | Niraj K. Singh            | Bodhgaya, Nalanda, Patna
 204 | Ravi Rocks                | Bokaro
  19 | Ian Lotriet               | Cape Town
  15 | Ivy Almacin               | Cape Town
  38 | Dr Brahm Prakaah Tripathi | Delhi
 130 | Virendra Singh            | Delhi
 271 | Satish Jain               | Delhi
 110 | Vikas Agarwal             | Delhi
 114 | Devi Singh Rathore        | Delhi
  58 | Dilip Singh Chanpawat     | Delhi
  95 | Anam Kumar Dhasmana       | Delhi
  51 | Gopal Sharma              | Delhi

使用偏移量 20 返回再次运行查询：

 id  |       display_name        |    city    
-----+---------------------------+------------
  95 | Anam Kumar Dhasmana       | Delhi
 114 | Devi Singh Rathore        | Delhi
 252 | Tarun Pratap Singh        | Delhi
 258 | Rajesh Kumar Pal          | Delhi
 255 | Chandan Singh Shekhawat   | Delhi
 268 | Amit Kumar                | Delhi
 100 | Ketan Mehra               | Delhi
 286 | Vikash Poonia             | Delhi
  61 | Belinda Schempers         | Durban
  67 | Pieter Janse Van Rensburg | Hoedspruit
 140 | Dr Hari Krishna Somanchi  | Hyderabad
 197 | Sstya Prabha              | Hyderabad
 118 | Dalpat Jodha              | Jaipur
 253 | Yash Shekhawat            | Jaipur
 120 | Govind Sharma             | Jaipur
 257 | Abhimanyu Singh           | Jaipur
  99 | Ghanshyam Singh           | Jaisalmer
 308 | Nitin Lobo                | Jodhpur
 124 | Rajendra Singh            | Jodhpur
  55 | Umed Gehlot               | Jodhpur

从输出中可以看出，在第一个查询和下一个偏移量为 20 的查询中都返回了一些重复项（参见 ID 为 114 的用户）。

使用 distinct 似乎工作正常，但为什么它在按相关表（用户配置文件）上的字段排序时返回重复项？

显然，如果我按 user.id 订购，那么它似乎也能正常工作并且不会返回重复项。

这里 user 和 userprofile 之间的关系是一对一的，没有 user.id 在 userprofile.user_id 中被多次引用（由框架（django）强制执行）。