The time needed to complete an entire project will always be greater than the sum of the estimates for completing the individual tasks.
Some necessary tasks will not be included in your detailed estimates.
Estimates are usually based on the assumption that things will go well, and they often don’t.
The time needed for communication and coordination is probably not included, and this work often increases exponentially as the entire project team size increases.
The time needed for requirements collection and analysis, design, testing and governance/compliance activities is often missing, or understated.
The time needed to bring new team members up to speed is often not included.
Significant productivity differences between developers are often not taken into account.
One way to deal with this problem is to add some additional reserve to the project budget that is under management control. This might generally be referred to as the Management Reserve (MR).
In some environments, there might be a distinction between contingency reserves that are established to address specific risks already identified, vs. a more general management reserve held back to address situations that have not yet been foreseen. Using the terminology popularized by Donald Rumsfeld, these might be used, respectively, for the “known unknowns” and the “unknown unknowns.”