There are a few reasons for this one of the main reasons is meetings. Have you ever been a part of a IT Project where you did not have to go to any meetings?
If you are in a meeting You are not coding
But What About…
Using the Restroom
Eating Lunch
Taking a Break
Talking to your Cube Mate
Attending B-Day Party
Not Feeling Good
Answering the Phone
If your plan counts on your people working on the project 8 hours a day every day it will always be off