In FlashPrint there is an Extrusion Ratio setting. It's set to 97 % for the stock PLA profile and 107 % for the stock PETG profile.
I would've thought these would be 100 % by default (and the same for all materials). Why is it that these defaults make sense and they differ? Why would you need to push more PETG for the same print than PLA if they're the same diameter filament?
If your extruder uses spring tension to hold the filament against the hob, the effective diameter of the filament at the point the hob contacts, which determines the amount moved, is a function of the compressibility of the material. Thus, it makes sense to use a higher flow ratio for more compressible filaments. However, this is very much a matter of what type of extruder your machine has, and doesn't apply if you have a tension arm holding the idler bearing at fixed distance from the hob.
I may be entirely out of my league here, But I think it's related to Viscosity, or how thin or thick the fluid plastic is while hot and Temperature Expansion, how much larger in volume the fluid plastic will be hot compared to cold.
