From Dark and Darker Wiki
Healing potions heal at a certain tick frequency for each rarity. If you have negative buff duration, you get fewer ticks and therefore fewer heals. The healing amount shown is for the base buff duration, 20 HP over 20 sec at base, or 1 HP/1 tick = 1HP/1 sec. With lower buff duration, you lose some ticks, so you lose out on some healing.
You can read more details on the Damage page (Healing over time section) and the Healing page.