Mutated from Update in informatica

When you have update override in the target and go for treat rows as update and target level properties as update else insert and insert .

This is an informational message stating that your session has inserted 411 rows of which 411 (all) were intended to be updated in the target. But none of those 411 rows were updated (no row with the matching key was found in the target) so their "row-type" has "mutated" into an "insert".
That's it. It's a normal operation.

Refernce link :http://technet.informatica.com/node/111332