NOT NULL after import

I just did an import. After the import I compared the database structure of the source db (where the export was taken from) and the target db (where I did import into).

There's a couple of tables where a few of it's column got NOT NULL after it got imported into the target db (where it did NOT have NOT NULL in the source).

I haven't done very much exporting and importing. Is this common occurence and an someone explain why.

BTW: Verison is 9205.

What type of import did you do?
Were the structures in place prior to the import?

I did a full export from the source.

Then I did an import schema by schema (using fromuser-touser, becuase I didn't want to import certain schemas).

And NO, no structure (other than the default from a clean installation of a new instance) were in place in the target db prior to importing.