High "transaction rollbacks" value in v$sysstat

I just noticed in a 9.2.0.6 instance on Win2K3 that statspack is reporting a 
lot of rollback in the "transaction rollbacks" statistic, but a much more 
reasonable (smaller) value for "user rollbacks":

Statistic                                      Total     per Second    per Trans
--------------------------------- ------------------ -------------- ------------
transaction rollbacks                      3,640,974           31.6          1.3
user calls                               135,007,494        1,172.0         47.4
user commits                               2,845,562           24.7          1.0
user rollbacks                                    21            0.0          0.0

I searched Metalink for bugs but didn't find anything.  Is it even possible for 
these values to be correct?  My understanding is that 'transaction rollbacks' 
is a subset of 'user rollbacks', i.e. URs is incremented everytime a user 
issues 'rollback' even if there is nothing to actually be rolled back, but TRs 
is only incremented when actual rollback work is performed.  Is my 
understanding incorrect/incomplete?  If so, what could be the cause of all this 
rollback activity?

Thanks,
Brandon

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