I just recalibrated my powermeter. Looks like the slope drifted from 21.78 (rounded to 21.8) to 21.37 (rounded to 21.4).
Because torque and cadence = a certain power, torque is calculate by dividing by the slope – which means my PM was underreporting by a couple of %.
SRM recommends recalibrating every six months or so – so I need to repeat this again in December or January.
Since the SRM uses the slope, combined with the zero-offset, combined with cadence and torque (think y=mx+b) while riding to generate a power number you want to recalibrate the system occasionally to ensure the slope is accuarate.