In Case Activity export custom fields attach to the activity type its not configured for

Custom fields attach to the other activity types as well where these are not configured (Find Cases > Export > Select Fields for Export)

Replication Steps:

1. create case type
2. add 2 or 3 activity types
3. add custom field to one of the activity types
4. create new case
5. add data to case activity, add data to the custom field
6. export out fields - case id, activity date/time, activity type, custom field that added above
7. the custom field gets attached to the activity type for which it was not configured

See Attached. Custom Fields are only configured for Activity Type1, but attached to Open case activity type and Activity Type2 as well.