Why didn't my pencil markings burn off?

Hi pottery community!

I have just bisqued some works to ∆05 with some underglaze on them but was surprised to find that the markings from my pencil are still visible on my work. Does anyone have any idea why these didn't burn off? I'm hoping to add some glazes to them do the final firing but before I do, I want to know if these little pencil lines are permanent or if ∆05 (somehow) just wasn't hot enough?

Thanks!