To reproduce:

Add a RadRichTextBox and a RichTextBox to a form. Set the AcceptsTab property of the RichTextBox to true. Start the application in the RichTextBox type:

1 *Tab* 2 *Tab* 3 *Enter*

12 *Tab* 123 *Tab 1234

Now try the same in the RadRichTextBox.

You will see that the numbers on the second row are not aligned.

Resolution: 
This issue is addressed in the new version of the control - RadRichTextEditor. Please use the new control instead the RadRichTextBox.