Your best guesses as to how various relevant factors are likely to affect the duration and cost of specific tasks, and of the project as a whole, are your project assumptions. Some, perhaps most, of the factors you must take into account will be outside of your direct control.
For example, the clockwork project manager might assume that all the exhibit designers will be available for the month scheduled to complete the exhibit design. But if half the team were to come down with the flu for a week or be assigned to another project, that assumption would be false and either the project scope or schedule would have to be adjusted.
It’s best if you state your assumptions clearly, right at the beginning of the project, and have others involved in the project review them. Together, you can discuss alternative scenarios and develop contingency plans. For instance, "If the supplier can’t deliver the materials required to do Task A at its scheduled time, then let’s do Task B before we do Task A."