Duct sizing chart - pressure loss

[TSN79]

I found this sizing chart for ventilation ducts. Notice the gray area for what I suppose is the accepted pressure loss area. What I wonder is why it changes as the sizes pass Ø800. From here on the upper pressure loss limit goes down to 0,7 Pa/m. Is this because of velocity issues or something else?

 

[russ_watters]

That's something I should probably know, but I don't. It could be an issue of duct strength, economics, noise. I'm really not sure. I use a duct sizing calculator (rotary slide rule, basically) which has no similar markings. where did you get it?

 

[TSN79]

Where did you get it?

Just googled around for it. Got it here:
http://dc169.4shared.com/doc/ZmDGPo2M/preview.html

 

[Travis_King]

I'm pretty sure it's a limit on velocity, and not on pressure losses. 200,000 l/s of gas rocketing through a pipe at 40 m/s is not within typical design limits.