AFAIK this way of measuring was introduced almost a year ago, in the beginning of the project. It was good for a while, but after there was a big progress in the last months and the n values increased somewhat, the correction (the 'c' in cEM) shows its deviation - which gets bigger the greater the difference between to n's is.
A new measurement will be introduced in the next client. Louie said it'll be Flops.