A combobox may not show all the expected entries.
The problem is related to the quantity of values returned. The Display Form makes a query to the ArcGIS server service to get the values, then removes the duplicates, and sorts the results to display in the drop down. The query will return a set limit of results, which may or may not include all required entries. The default setting in ArcGIS is 1000 records. One thing you could try is increasing the Max Record Count on the map service so that the query to populate the drop down returns more records allowing the Display Form to get all unique records without missing any. However, the preferred method would be to swap out the combobox for an auto-complete box. A combobox is ideal for a small list of items but an autocomplete box should be used for anything more than 100 values. It's a cleaner user experience and it performs a little bit faster since the query to the map service to get the values will already be filtered by couple of characters and less records are returned.