Strange Behaviour of 'GROUP BY' clause when used without 'ORDER BY' clause

Using GROUP BY clause without using 'ORDER BY' clause

 A table named HOSTEL(hostel_no NUMBER, no_of_stud NUMBER)  is created and populated with values as follows : 

--------------------------------------

hostel_no|     no_of_stud     |

--------------------------------------

1001       |             124          |

1001       |             234          |

1002       |               97          |

1002       |             134          |

1003       |             234          |

1004       |             123          |

1004       |               89          |

1001       |               67          |

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When GROUP BY command is applied as : 

SELECT hostel_no, sum(no_of_stud) as total_students FROM

HOSTELS GROUP BY hostel_no;

, and the output comes as:

--------------------------------------

hotel_no  |     total_students |

--------------------------------------

1003        |            234           |

1001        |            425           |

1002        |            231           |

1004        |            212           |

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Now, the questions arise : 

• Why the hostel_num 1003 came at the top ?
•  Shouldn't it come at row number 3 ?
•  What is the logic behind this kind of order ?

It is  known, that GROUP BY clause consolidates the repetitive column values and group them together. Now the doubt here is: How it decides the order of displaying those values, as here, one would expect an output like this :

--------------------------------------

hotel_no  |     total_students |

--------------------------------------

1001        |      425                 |

1002        |      231                 |

1003        |      234                 |

1004        |      212                 |

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