We just added a repeat attack bonus on the world boss to give additional XP and cash based on total attacks to give all the incentive to go above and beyond the Attack Achievements and Reward Items.
We will be actively tweaking the bonus, we may have set it a little low to start but will increase it after it has some time in the wild.
You will get a bonus XP and coins after each attack, which will add to your overall XP and coins earned from that fight which will increase the amount collected after death as well. If you didn't get any attacks under the new change, you will not see any increase until the next boss battle. Does that make sense? Even if you got one attack in, you would have seen the bonus for that specific attack but it wouldn't have impacted the total collected from death as fighting a new boss from scratch with this change.