The input signal is split into three frequency-bands. Each band is passed through a stereo compressor / expander, where gain and processing can be altered.
The outputs of the compressors / expanders are then re-combined and fed to the outputs.

There are four different frequencies for each of the two crossovers that can be separately selected for the low-to-mid and the mid-to-high bands.

The three frequency-band compressors / expanders have separate controls so that different parts of the audio spectrum can be adjusted and tuned.

With separate gain controls for each frequency-band, the MLA-4 can also be used as an equalizer. You can always adjust the controls for the separate frequency bands to different settings.

There is no technical reason why they should be the same for all frequencies.
Separate Input and Output gain controls, make the MLA-4 easy to use for different operating levels.