Full table scan - high water mark
Oracle (as well as any other RDMS) doesn't scan "rows". It operates with blocks (other vendors may use different terminology , e.g. page) , and it doesn't know which blocks have or don't have rows. Also, sequential read is much faster than random , and it's way cheaper to read more than needed into memory...  A good graphical explanation of HWM : http://chandu208.blogspot.ca/2012/02/high-water-mark-hwm.html