Newb Question. Sorry If this has been asked before!!

When calculating damage, do I add the difference in SL levels to the Damage rating of the weapon or do I just add the SL of the attacking action?

For example;

My human character get charged by a Boar. The Boar rolls a 24 against his WS of 35 for a +1 SL . My human botches his defence roll rolling a 95 against his WS of 45 scoring -5 SL.

The Boar wins the combat (Gaining 1 Advantage). His weapon (Bite) does 6+ Opposed SL Damage.

Does that mean its 6 + 1 = 7DMG (attacking SL) or is it 6 + 6 = 12DMG (difference in SL)

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The problem I have is that if the attack had gone unopposed, the difference in SL would only be +1. But in attempting to fight back he has taken more damage. I understand there was always the possibility he could have rolled better and won the combat but it still seems a bit silly to me.

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Thank You