http://docs.moodle.org/24/en/Calculated_question_type contains such description:
Adding individual values to the list is easy:
1. In the 'Param' field for each wildcard, enter the value you want
2. Scroll down to the 'Add' section and click the Add button (leaving the number of items set to 1)
3. Repeat the above steps as many times as necessary (the maximum number of items is 100)

When I try to add values in such way newly added values are not the same as was in 'Param' field for each wildcard. On every "Add" button click randomly values from specified range are generated.

Tested on Moodle 2.4.3+ and demo.moodle.net (2.4.1+)