Yes, the issue we are all pointing to is NONLINEARITY of input vs. amplifier output (besides other parameters and even human perceptual factors).
I'm *guessing* that FAS's relative amp loudness might be constrained by the dynamic range of the amp fully dimed and hit with a hard input so as not to get anywhere near digital clipping. FAS's cleaner amps do seem to be quieter at low-moderate settings, and it's likely that they are more linear and will get relatively louder as settings are cranked: witness a Twin Reverb. Highly compressed/saturated amps have a lower slope or a roll-off of output relative to input. Thus for processing sake their internal "volumes" may be set to be as high as possible without digital clipping to maximize quantization resolution. Or there is some entirely other valid explanation only FAS knows!
But regardless: even if FAS used some method to set them "close" at "default" settings with an assumed certain input level, all the other factors still affect the perceived/actual volume: hotness of guitar PUs, finger/picking attack, bass cut, etc...