sharepoint list item without entry in required fields

i have a sharepoint 开发者_JAVA技巧list which has few rows without having any entry in required fields .

are there any ways to populate a item without required fields?..i need to know the reason how those items were made up without required fields.

could some one please tell me how this could have happened.

In addition, I don't think all of those checks are enforced when manipulating items via the object model or web services.

• If the column was optional when the item has been uploaded/created.
• If you uploaded a xls spreadshead
• If you uploaded multiple documents at a single blow