Is Biodiesel not a hydrocarbon? The typical petroleum product would have a C(n)H(2n+2) structure. Is the stoichiometry that different for Biodiesel that there can be a resultant 48% reduction in CO2?
Doesn't the concept that fuels which are 'grown' are 'carbon neutral' require one to conclude that the earth's plant life can soak up any additional CO2 produced by any means? Isn't that CO2 soaked up in additional plant life regardless of whether or not we then convert that plant life back into a fuel?
You are behind the curve, modern thinking is that old oil from the ground is new CO2 and bad, but new oil from current crops doesn't have to be counted because it is from the CO2 presently in the air and emissions are a net zero.
I guess the actual measurements will have to be adjusted for this if they don't show a true reduction over time.