Well for me having made the jump from IC to being responsible for my whole team, the issue is that the promises I used to give were based on my ability, whereas now I have to take the whole team into account. And so sometimes, I'm left saying to another team, it's going to take X weeks to do this, even though if I were doing it, I think I could do it in Y < X weeks. But I have to face the reality that my team is less senior than I am and so will take longer, as I train them up.

Moreover, when I did overpromise, as an IC I would put in the extra hours to deliver, so my estimates always seemed to my superiors to be 'on time'. But now as a manager, I can't force my team to work extra hours (and I don't wanna be that guy), so I have to take that into account when making promises.

Ultimately, it's the difference between being responsible for myself, and now being responsible for others, not only responsible for the tasks they do, but also for being responsible for communicating the ability to do it to other teams.