Hi Kaushal,
I think the problem is that these "default values" are only applicable within the scope of the geodatabase, that is, software by ESRI that is aware of the geodatabase concept, like ArcGIS.
If you use other software to update these fields, they are probably not able to use / don't "see" the default values, as these are stored in the ArcSDE / Geodatabase system tables that are ignored / inaccessible by the other software.
You may need to define defaults at the database level instead of through setting them in ArcGIS, or have some database triggers on your table to automatically set Null values to a default.