... the total strenth must be equal or higher the total strength of the invaders strength in that rampart.
This is not correct, though the rules really don't help. The relevant text in the Note on p16 should read: "To kill any Invader's Units, the Defender's Volley's Strength has to be equal or higher than the Invader's Unit's Strength". The example on p17 clarifies this.
In fact it would be better as: "An Invader's Unit will not die unless it receives Fire Strength equal to or higher than its Strength." And for clarification the rules already say that the total Fire Strength determines the total Strength of Units killed.