How to understand ADHDer who is "on" sometimes and "off" other times?

A lot of people have shared the experience of a partner with ADD whose focus and attention is fairly consistently "on" at work or maybe in some social situations, but somehow switches off when they are at home.  It has a few interesting consequences ...

- It adds to the difficulty of diagnosis, and of persuading anyone (including them) that the issues are real

- It more easily creates the feeling that the person could be more attentive, etc, at home if they wanted to be. So they seem to be choosing to be careless, thoughtless, lazy, etc. with us.  It's easy to take that personally, and seems to require near sainthood to remind ourselves 24/7/365 that it's the ADHD.

It's hard to explain and understand this.  Has anyone come across a good discussion of this topic in the books or articles you've read?