One way we can view tasks and the associated variable levels of attention as different Tetris blocks. As you go through the day, each task acts as a (generally) unique Tetris block that must go into your space with the goal of maximizing the filled space versus gaps.
On good days, and especially when you can control the work you're tackling, you can optimize the order and delivery of these blocks with a planned order, making it easy to put things on the board.
On bad days, either you couldn't easily plan things out or blocks you haven't planned for (random new tasks, etc.) show up in your queue and throw things out of balance. These days are when you are scrambling around just trying to keep things from blowing up and leaving too much empty space.
In addition to being Tetris blocks, there is a finite amount of energy each day you have to rearrange and align the different blocks as they come up. Larger blocks (e.g., bigger, more stressful projects) take more energy to move than smaller blocks (e.g., email tasks). Ideally, you have some idea of the tasks you need to do the next day so you can setup your queue to minimize the amount of rearrangement (and thus energy usage) you go through during the day.