In my personal experience this is 100% right. But the reason could be the level of pressure and the anxiety involved.
On manual jobs I can work more than 8 hours a day with a good productivity and without feeling tired. In software development I can't. Probably this depends on the pressure. If you are in a manual jobs with higher productivity pressure (like assemble <n> units a day where the average is less than that) will have a similar effect.
Also I find a difference between people who have an expertise in a more specific software development field (SQL databases) than on people who jump to different problems/languages/technologies in different computer fields all the time. You can maintain your productivity if you narrowed your areas of expertise.