When does kb_uc_can_read_mtom have no user criteria for a kb?

[TR Vishwanath]

For some knowledge basis even though the UI shows "can read" user criteria configured, querying kb_uc_can_read_mtom using the table API returns an empty array. For other knowledge bases the api returns a list of user criteria. 
What is the reason the API returns an empty list of criteria from this table for some knowledge bases?

[TR Vishwanath]

I wanted to clarify my problem:

I see that querying the kb_uc_can_read_mtom table using the table API returns the correct user criteria value for 2 of the knowledge bases but returns an empty array for the 3rd kb, although the UI shows a can-read criteria for that kb. Wonder why the API is returning an empty result