Q -Why does the BCL-BCR guide have the older EBC color values?
A- Actually the guide has both or either depending on your preference.
Review Early on the British Institute of Brewing (IOB) and the European Brewing Convention (EBC) use different methods for quantifying the color of beer and malt. Even after 1950 when the American Society of Brewing Chemists (ASBC) adopted the standard spetrophometric measurements of beer color using 1/2' samples and a light wavelength of 430 nm (ultrablue or ultra-powderblue), the other IOB and EBC were using different methods and a light wavelength of 530 nm (Green). These methods were not linearly related but generally the conversion formulas
SRM = 0.377 EBC + 0.45 and EBC = 2.65 SRM 1.2
Since 1991 the EBC uses a 1 cm sample and a wavelength or 430. So now the ASBC SMR is measured for ½ at 430 nm with its corresponding absorbance multiplied by 10. While the EBC is measured for a 1cm sample at 430 nm but its absorbance is multiplied by 25.
So todays conversion is much simpler -
SRM X 1.97 = EBC and EBC / 1.97 = SRM
However many maltsters and brewers still, especially in the US, use and reference to the older SRM-EBC conversion. So for convenience and to avoid some problems the older EBC values were used hard to break old habits. SRM is primarily seen in the US for beer and L° for grain. However since the Lovibond measurements are more subjected to human error more grain values are being presented with color references of the ASBC SRM. The default guide for the time being is the older EBC values.
However #2 The BCL BCR guides are also available with the newer, subsequent to 1991, values on them for those, especially in Great Britain and Europe who would prefer them.
Hopefully with time and more beer consumption a common system will come about.