These choices are automatically pulled from the choice values in the incident impact, urgency, and priority fields. If you want to add additional choices, you need to right-click the field labels on the incident form and select 'Configure choices'.
In my system, the dictionary entries for the choice lookup fields (on the lookup table itself) aren't pointing to the correct choice field. You'll also want to verify that these fields point to the matching field on the incident table to pull their choice value.
