Time to print same item varies

Hi, I am using an Ender 3 V2 with Jyers firmware and Prusa Slicer 2.5. Over the weekend I printed a, what Prusa Slicer estimated to be a 11 hour print. It was actually just over this time. Immediiatelly after I printed the same file again. Same filament, same file, same settings, absolutely no changes. This print took just over 13 hours.
Has anyone any ideas about why the same print took an additional 2 hours? As I had timed it so that I could go to bed at my usual time, I was not happy!