The 7-series recorders show a preamplifier gain value on the LCD. This value is an approximation of the mic preamp gain. It is not an exact representation of input-to-digital signal gain.
It is impossible to make this match the exact gain of the preamplifiers. The 7-series recorders use an all-analog mic preamplifier for maximum signal-to-noise, dynamic range and lowest possible distortion. Being analog, the gain of the preamp is determined by the potentiometers on the front panel. Because all audio-taper potentiometers have a fairly wide tolerance, it is difficult to match the exact gain versus rotation of the pot on a unit-to-unit basis. The preamplifier's gain increases faster per a given rotational input at the very end of the potentiometer's rotation because of the mic preamplifier topology. This gain value is especially difficult to estimate numerically.
Beginning with 7-Series firmware version 2.15, Sound Devices recalibrated the stored curve of the displayed numbers. Neither the actual gain amount nor the curve of the gain verses rotation changed at all, only the displayed gain value changed. This revised gain curve display more closely matches the actual analog gain. As stated earlier, it will not match it perfectly.
Because the line inputs on the 7-series use a completely different circuit to control the gain—analog switches, not potentiometers—the displayed gain is fairly accurate for these inputs. Again, due to manufacturing tolerances, the numbers will never be an exact match of gain to display.