This is just speculation but the difference could be attributable to (i) resume drops and (ii) regional interview fairs.
(i) A number of fims (UK firms come to mind) have resume collections and a contact person but don't actually do any OCI interviewing. Although I assume they do callbacks at their offices.
(ii) UT Students participate in a number of regional fairs summer/early fall. These employers would probably be listed on NALP as interviewing UT students but wouldn't come to OCI.