The nearly equal is an assumption, and their magnitudes are not well understood either. With those assumptions, a net increase from anthro sources would be valid. But obviously the mechanisms and precision for CO2 as a whole is difficult to pin down, so that's why they use the isotopes. The trouble is, I have yet to see any isotope analysis that yields any numbers for the human component. Your favorite site for example: http://www.realclimate.org/index.php?p=87 goes on and on about how the decrease in ratio is "exactly what we expect if the increased CO2 is in fact due to fossil fuel burning" without any numerical analysis to back up that statement. Zip.
It's not an assumption, it's an estimate, with the requisite error bars. But the estimate is based on analysis of data -- sometimes direct measurements, other times "economic" analysis. The air-sea flux is VERY well constrained by thousands of at-sea measurements, for example.
The trouble is, I have yet to see any isotope analysis that yields any numbers for the human component.
I found a few papers that allude to it, but its wrapped up in a bigger picture presentation and its complex. I'll keep looking. Looking...
I assume you found/saw this:
http://www.grida.no/climate/ipcc_tar/wg1/108.htm (can't wait to see the corresponding section in the new report!)
Here's a couple of references to peruse:
Isotopic Simple Global Carbon Model: the Use of Carbon Isotopes for Model Development (real interesting conclusion in this!)
Science reference, sorry, but maybe you can get it:
Oceanic Uptake of Fossil Fuel CO2: Carbon-13 Evidence (April 3 1992 issue)
Global Carbon Cycle (this is real simple, but the data in the lower-right hand corner of Figure 7.02 might be what you're looking for)
http://www.holivar2006.org/abstracts/pdf/T3-032.pdf (neat poster -- actual number, del13C decreased 0.44 per mil from 1979-2000, annual rate 0.0021, similar to Quay et al. referenced above)
Gotta keep this one in my pocket for drrocket: δ13C in CO2 at Mawson, Antarctica (wow, now I know how to do the del symbol!)
That's a start.