Each table is made up of extents and each extent is made up of oracle blocks - a common block size is 8k. So you have a table with 10 extents (80K).
You populate your table with 2 million rows of data and you will have many hundreds of extents. Now lets assume that you delete over half of the records in the table. Oracle still has the same number of extents but many of the blocks are empty. When you run a query against the table Oracle will scan through all the blocks including empty ones looking for data. So you can think of the total number of extents / blocks used as the high water mark.
To fix you export the table, drop it and import it back in.
Yes, after deleting rows the high water mark is the same. New rows will use the space left by those that were deleted unless they don't fit into that space then new space is allocated and the high water mark increases. To reduce the highwater mark export / datapump the table, drop it and import it back in again.
If you have to remove the whole data,then in that case Truncate command will be better then Delete because it will release the memory and the High water mark will be resetted to the original value.Hence later on, fetching on that table will be faster.