question about cx_Oracle .thanks

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Hi 
  i hava a db ORACLE 10G,a table valid_card
(card_no,varchar2(10),now_balance number (12,2))

 its'some record in table ('7188','42055.66')

 i use cx_Oracle to select now_balance from table
 
  curobj.execute("select loan_amount from valid_card where
card_no='7181930166881974'");
[<NumberVar object at 0x013A6920>]
>>> tuple=curobj.fetchone()
>>> tuple
(42505.660000000003)

why 42505.66---->42505.6600000000003???
thanks