explain plan shows fts on same table multiple times with different costs

Sorry if this is a simple question.
It may be relevant to all versions, but this one comes from 10.2.0.4

The statement reads from the BKMAP_CONTRACT_LINE 3 times in this statement all using full table scans, but the reported cost for each fts differs

Is this usual?
Is the difference in cost caused by dbms_xplan being aware of the likelihood that many of the rows will already been in the buffer cache (presumably from the largest fts - Id 13)

I expected to see the same cost for each fts on the BKMAP_CONTRACT_LINE - hence the reason for my question

thanks
Craig

| Id  | Operation                 | Name                      | Rows
| Bytes |TempSpc| Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT          |                           |
550K|   222M|       |   150K  (6)| 01:22:39 |


|*  1 |  HASH JOIN RIGHT OUTER    |                           |

|   2 |   INDEX FAST FULL SCAN    | BKMAP_CUSTOMER_SITE_N02   |
1601K|    18M|       |    75  (14)| 00:00:03 |


|*  3 |   HASH JOIN OUTER         |                           |

550K|   216M|   180M|   148K  (6)| 01:21:13 |


|*  4 |    HASH JOIN              |                           |

550K|   173M|   157M|   117K  (6)| 01:04:32 |


|*  5 |     HASH JOIN RIGHT OUTER |                           |

550K|   151M|    17M| 82844   (8)| 00:45:25 |
|   6 |      TABLE ACCESS FULL    | BKMAP_MATERIAL_ITEM       |
781K|  9162K|       |   231  (19)| 00:00:08 |


|*  7 |      HASH JOIN OUTER      |                           |

550K|   144M|   130M| 80959   (8)| 00:44:23 |


|*  8 |       HASH JOIN           |                           |

550K|   124M|   150M| 66175   (7)| 00:36:17 |


|*  9 |        HASH JOIN          |                           |

820K|   140M|    17M| 50570   (8)| 00:27:44 |


|* 10 |         HASH JOIN         |                           |

310K|    13M|       |  5205   (5)| 00:02:52 |


|* 11 |          TABLE ACCESS FULL| BKMAP_ORG_TO_GEO_XREF     |     3

|    27 |       |     3   (0)| 00:00:01 |


|* 12 |          TABLE ACCESS FULL| BKMAP_CONTRACT_HEADER     |

5957K|   210M|       |  5174   (5)| 00:02:51 |


|* 13 |         TABLE ACCESS FULL | BKMAP_CONTRACT_LINE       |

15M|  2012M|       | 23742  (14)| 00:13:01 |
|  14 |        TABLE ACCESS FULL  | BKMAP_CONTRACT_REFERENCES |
15M|   866M|       |  3799  (13)| 00:02:05 |
|  15 |       TABLE ACCESS FULL   | BKMAP_ITEM_INSTANCES      |
13M|   512M|       |  6873  (20)| 00:03:47 |
|  16 |     TABLE ACCESS FULL     | BKMAP_CONTRACT_LINE       |
23M|   974M|       | 20994   (3)| 00:11:31 |


|* 17 |    TABLE ACCESS FULL      | BKMAP_CONTRACT_LINE       |

8009K|   618M|       | 21715   (6)| 00:11:55 |
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