Before any system is handed over to users for Acceptance Testing, the system suppliers and/or developers have tested it thoroughly at various levels.  These tests are usually technically-focused and only one part of the Acceptance Testing process.  The IT department will also have input into Acceptance Testing based upon technical Acceptance Criteria and report to the Project Manager.

Unfortunately, the computer professionals will not understand your business as well as you do.  Computer personnel will view the system from a technical rather than business-user perspective.  Given they had a hand in constructing the system, they may have developed misconceptions of how it should operate.

For these reasons it is best for end-users to Acceptance Test their own system.