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By default, ArcGIS Server map services limit the number of records returned by a query to 1,000 records. You can raise this limit in the Service Properties dialog box. The limit is important to remember when configuring the task because if you click the Get Sample Values button, only the first 1,000 records will be scanned for unique values.
It sounds like you're populating your ComboBox with the result from a query task.
The 1,000 record limit is configurable, but 1,000 records was chosen to avoid accidentally returning thousands and thousands of records to a client which is likely to slow down any application.
More info in the server help:
heres a link to the ArcGIS Server 10.1 help which explains how to set the "Maximum number of records returned by the server:"