I work for Circular Software and can tell you why. We are aiming our CircularFLO tool at publishers with existing print documents of any size and we don't assume any prior knowledge of viewports / pixel dimensions etc.
Instead we us the page proportion and adapt to 2 megapixels area (this area can be altered in the boilerplate, see our instructions page)
I will suggest we make an adaption to a forthcoming update whereby if pages have been designed using pixels rulers (instead of inches or mm for example) we offer to keep to the pixel dimensions exactly as the user has set.