For every flight it computes the CO2 actually emitted and compares it with an ideal flight: same aircraft type, direct great-circle route, the most fuel-efficient altitude and speed profile for that distance, and the same real wind. The difference is split into a lateral component — the route — and a vertical one — the profile.
It measures the distance from a theoretical optimum, not recoverable fuel: published estimates of what is actually recoverable are far smaller. Results are aggregated by route and airport only; nothing is published per flight, per aircraft or per operator.
A personal project, built with AI assistance. I am not an aviation professional, an air traffic controller or a climate scientist — the method is open precisely so that people who know the field can check it.