ORA-ROWSCN Changing Unexpectedly?

I am struggling with the ORA_ROWSCN pseudocolumn. (Oracle version is 11g). I am aware that without ROWDEPENDANCIES set on the tables, the SCN of a row is in reality that of the block in which the row is resides. The problem I am seeing goes something like this:

1) A datapump import creates a large table (400M+ rows) at 11 am in the morning
2) No subsequent changes are made to this table (at least that I know of)
3) Despite no changes being made the ORA_ROWSCN of almost all the rows changes every few minutes. (a few rows remain with SCN's indicating the original time of import, but the rest change)

I detect this behavior by running this query:

SELECT ora_rowscn, SCN_TO_TIMESTAMP(ora_rowscn),COUNT(*)
FROM test_table GROUP BY ora_rowscn

This ran around 5.15 am on 10/27 and resulted in:

72353518 27-OCT-12 05.17.21.000000000 AM 492009391
67515248 26-OCT-2 11.01.45.000000000 AM 370

Note that almost all rows have a very recent SCN. Only a tiny portion of the rows (370) have an SCN consistent with when the import completed. If I rerun the query a bit later, the SCN of the large rowcount will have advanced again.Block size on the database is 8192.