I'm guessing Lasciel was footing the bill for Hellfire usage for Harry since he never experienced the Soulfire slump when using Hellfire.

My theory for how harry's hellfire never ran out is this: soulfire runs off his soul and is replenished by Harry having feel good moments with other people according to Bob. What if hellfire also runs off of the soul but is replenished by Harry having angry moments? Like if it regenerates every time he uses magic in anger or something