You'll find the diagram is a guide, and it's important to remember that should you be running a site of job, that you are responsible to manage, monitor and organise the site, job and therefore lifts. Bearing in mind that should you not do that, and subsequently injure a site operative they can bring action against you - always one eye open to being sued or HSE actions.
Employing the worlds strongest man and telling him he cant lift a certain weight is stupid, therefore the diagram is a guide and any manual handling task should be assessed for weight, load type, person, environment etc.