When is row index and full table scan done exactly???

i have a table with 1lacs record and when i perform a delete on 60thousand record it does a index scan and even after that if i fire any select query on even rest of the records it always shows a index scan
whereas oracle docs says if the number of rows being fetched is more
 then 15-20% of the total number of rows it does a full table scan

so why does it not do the full table scan i have a normal index created on the table


i have also tried alter session set optimizer_index_caching=0; (suggested to me bt didnt help)