Tom, Scrum is changing over time, too. The initial commitment to deliver certain stories in the sprint is no longer valid. Instead, you are committing to the success of the project, with understanding, that if you discover anything that would jeopardize your project, if you don’t react immediately, you have to switch to this task, and don’t have to continue working on things, which are already of a lower priority, but you “committed” to them. I think it’s highlighted in a several newer books on Scrum, I would recommend this one in particular: “Scrum: A Pocket Guide” by Gunther Verheyen https://www.amazon.com/Scrum-Pocket-Companion-Practice-Publishing/dp/9087537204.