Redmine

I get a list of tasks with filters that have been paginated:
http://redmine_server/redmine/issues.json?status_id=10&tracker_id=7&cf_15= ><2017-09-25|2017-10-01&offset=0&limit=25
I get 25 tasks.
Next I get the following 25 tasks:
http://redmine_server/redmine/issues.json?status_id=10&tracker_id=7&cf_15= ><2017-09-25|2017-10-01&offset=25&limit=25
In the second result, I get one or two tasks, which are in the first result, why?

Environment:
  Redmine version                3.1.0.stable
  Ruby version                   1.9.3-p484 (2013-11-22) [x86_64-linux]
  Rails version                  4.2.3
  Environment                    production
  Database adapter               PostgreSQL
SCM:
  Git                            1.8.3.2
  Filesystem                     
Redmine plugins:
  redmine_agile                  1.3.11
  redmine_checklists             3.1.1
  redmine_contacts               4.0.1
  redmine_issue_history          1.0.0
  s1                             0.0.1
  scrum                          0.12.0