Mega Hydreigon has Lernean ability which divide power of attack depend on how much its heads. This part has no trouble.
The problem laid on when Mega Hydreigon uses Dragon Rage. Dragon Rage deals true damage and what i could see, it was intended to deal 40 damage each head and the true damage not suppose to involved with Lernean divided power. When Mega Hydreigon has 5 heads, it does exact damage which is 40*5= 200 damage. But when its HP reach half and turned to 6 heads or above, the true damage actually DIVIDED and it won't reach 200 true damage like what 5 heads Mega Hydreigon did. Logically speaking, it should be 240 true damage when it reach 6 heads or 360 true damage when it reach 9 heads.
I am not profound on english so I will add ss to add my explanation.