Carbeds are crash tested, but they are not held to the exact same standards as a normal carseat. Some of the exemptions from testing are because it just wouldn't make sense with a carbed (for example, no downward rotation component). I haven't looked carefully at FMVSS 213 to see if there are other exemptions that may lead me to believe that a carbed may be less safe than a carseat for a child who will fit either. I do know that the incline of a carseat is safer than a car bed in a crash, since the shell takes the force and the harness is just holding baby in the shell, whereas in a car bed the harness is doing a lot more work. But for a baby who needs the lay-flat of a carbed due to airway issues, of course a car bed is safer.
I don't know how manufacturers come up with their bottom weight limit. I haven't seen anything in FMVSS 213 that says that a seat rated below 5 pounds is tested differently than one rated from 5 pounds. I haven't looked carefully for that detail, but I haven't seen it during general reading.