Calendars work when other people are depending on your decisions, and it's basically more important to make a decision than anything else. Everything is driven by the need to make that decision. The decision may be imperfect, but the overall system can tolerate that most of the time - and when it cannot, it appears as a schedule problem.
Calendars fail when you need to perform labor to complete a piece of work, and you don't know how long it'll take. A to-do list is a detailed list of tasks to complete, generally in a specific order, to complete a piece of work. That work is probably part of a larger piece of work. There are dependencies between pieces of work. Times, if they exist in the to-do list, are generally estimates.
The only way to put tasks into a calendar, if you want the calendar to reflect reality, is to pad out each task with extra time. That way, the schedule doesn't fail.
However, if you padded out your to-do list with enough extra time so you could fit it into a calendar that could remain largely unchanged for the next year... your productivity would be pretty low. It would be a pretty relaxing job, however.
There are problems when these two different ways to organize work aren't reconciled correctly. You have work crunches, where people are forced to work late, or you have calendar failures where your obligations to outside parties aren't met, or both.
Both are intolerable, and a sign of bad management.