This happens because the "complexity" enumeration inserts numeric strings into the attribute.  To see what I mean, open your project area configuration and switch to the Process Configuration Source, search for /enumeration to get to the right area, then look for the 'complexity' enumeration.  It may look a bit like this OOTB scrum:

            <configuration-data final="false" id="com.ibm.team.workitem.configuration.enumerations" xmlns="http://com.ibm.team.workitem/enumerations">
                <enumeration attributeTypeId="complexity" name="complexity">
                    <literal default="true" id="0" name="0 pts"/>
                    <literal id="1" name="1 pt"/>
                    <literal id="2" name="2 pts"/>
                    <literal id="3" name="3 pts"/>
                    <literal id="5" name="5 pts"/>
                    <literal id="8" name="8 pts"/>
                    <literal id="13" name="13 pts"/>
                    <literal id="20" name="20 pts"/>
                    <literal id="40" name="40 pts"/>
                    <literal id="100" name="100 pts"/>
                </enumeration>

However, if you have added additional entries, say 71 pts it may look like this:

<literal id="complexity.literal.l4" name="71 pts"/>

This can be corrected but will be a mult-pass process so grab on an pay attention ;-)

1. For any entries that have id="complexity.literal.lX" change the name= to something eye-catching.  e.g. <literal id="complexity.literal.l4" name="NOT 71pts"/>
2. Create a new literal whose id is an integer value <literal id="71" name="71 pts"/>.  You can copy/paste additional line in the process source.
   
3. Save the project area.
   
4. Change work items whose story points show your eye catcher to the new entry.
5. Delete the miscreant entries from the enumeration
6. Remember this for the next time you need a new value.  It's ok to use the UI to enter it, but change the id= to the numeric value

Hope this displays right and helps.