Originally Posted by
canadiancow
The aircraft generally flies with a positive angle of attack.
If the seat is parallel to the floor, your head will be lower than your feet, which really sucks.
If the seat is perpendicular to gravity, you get what you want.
Can you clarify what you mean by "truly flat"?
Cow is correct,
The seats are not actually "lie flat" 100% by design, because of the angel etc, they are designed to be slightly inclined so they give you a feeling of lie flat.