Wouldn't the next node (17,null) be checked to ascertain the value of the first search key (referring to the diagram with green highlight) ? Otherwise, how will the program know when to stop accessing nodes in the backing store to retrieve the values?
Edit - I understand why it's the case - they've asked for range query with search key greater than or equal to 7 and less than 15. Thus after accessing the node containing 15, it won't check further as the range upper bound has been encountered. Thus only 5 blocks will be accessed.