Model Save And Automated Timestamps

I have a field in my database(MySQL) table of
type:TIMESTAMP with
default value:CURRENT_TIMESTAMP and
attributes : on update CURRENT_TIMESTAMP

Normally , when I update any field in the table (lets say by hand or via plain php code) , the Timestamp field gets automatically updated.This is normal MySQL behavior.

Although , when a model gets saved via Yii ( $model->save() ) the specific Timestamp field doesn't get updated.

Is the model->save() action not just a simple mysql update action ?
Any clues why this is happening ?