I would assume that your line has Interleaving/INP, and the 71Mb max attainable rate is calculated using the original attainable rate calculation method (as defined in G.993.2), which does not have to take into account the INP_min parameter, whereas the 63Mb max attainable rate has been calculated using the improved attainable rate calculation method, which takes into accounts lots more parameters including the minimum INP level required on the line. There is the g997ansg / G997_AttainableNdrStatusGet command to report the attainable rate method in use, although there does not appear to be a corresponding set command.