You don't work in full focus for the full hours. Other hours your are doing relatively low focus things such as code reviews, answering questions, eating lunch

You don't work in full focus for the full hours. Other hours your are doing relatively low focus things such as code reviews, answering questions, eating lunch, attending meetings, interviewing, talking about the direction and design of the project, etc. They want you around as an on-call 'consultant' so you can unblock other people more quickly. When it's just yourself green fielding a project, you don't really need to be anyone's consultant then, so you can just work the productive hours. Some companies unfortunately care about face time too :/