common table expression - PostgreSQL changes result when i play with offset, limit -

hi guys im using below query
if change limit 10 or 20 results different
i'm learning postgresql couldn't figure out problem
there postgresql specific warning ?

 recursive children (         select id, name, 1 depth         wapi_categories         parentid = 1 , visible = true         union         select a.id, a.name, depth+1         wapi_categories         join children b on(a.parentid = b.id)         a.visible = true         ) select wapi_issues.* (select distinct on(wapi_issues.publicationid) wapi_issues.* wapi_publications                                 right join wapi_category_publication on wapi_category_publication.publication_id = wapi_publications.id right join (             select *,                 max(wapi_issues.issue_identifier) on (partition wapi_issues.publicationid) max_id wapi_issues wapi_issues.status = true order wapi_issues.issue_identifier desc                                 ) wapi_issues on wapi_issues.publicationid = wapi_category_publication.publication_id                                 , wapi_issues.issue_identifier = max_id                                 right join wapi_issue_files on wapi_issue_files.issueid = wapi_issues.id wapi_publications.status = true , (wapi_category_publication.category_id in (             select id children         ) or wapi_category_publication.category_id = 1) , wapi_issue_files.pdf = true) wapi_issues order wapi_issues.issue_identifier desc         offset 0 limit 20  wapi_categories   ------------------ id, name, parentid   ------------------ 1, "root", 0   2, "child 1", 1   3, "child 2", 1    wapi_publication   ------------------ id, name, status   ------------------ 1, "publication 1", true     2, "publication 2", true    wapi_categories_publication   ------------------ id, category_id, publication_id   ------------------ 1, 2, 1   1, 3, 2    wapi_issues   ------------------ id, publicationid, title, issue_identifier, status   ------------------ 1, 1, "issue 1", 1, true   2, 1, "issue 2", 3, true   3, 2, "issue 3", 1, true   4, 2, "issue 4", 2, true    wapi_issue_files   ------------------ id, issueid, file, status   ------------------ 1, 1, "file1", true   2, 1, "file2", true   3, 2, "file3", true   4, 2, "file4", true   

expected result

simply want distinct wapi_publications , last issues sorting wapi_issues.issue_identifier

id, publicationid, title, issue_identifier, status   ------------------ 3, 1, "issue 2", 3, true   4, 2, "issue 4", 2, true 

what mean saying results different

when limit 10;

id, title   1, "a"   2, "b"   3, "c"   4, "d"   5, "e"   .   .   .   

when limit 20;

id, title 2, "b" 4, "d" 1, "a" 5, "e" 3, "c"   . . . 

i ended changing order clause this

order wapi_issues.id asc, wapi_issues.issue_identifier desc 

what understand postgresql needs explicit sortings