Why are some carriers' scheduled departures reported as 0 in the T-100 data?

The non-reporting of scheduled departures for certain carriers in the US DOT T-100 dataset is a known issue.

Our recommendation is to compare the T-100 Departures against Scheduled Flights (e.g. by running the Scheduled Monthly Summary Report, Diio Mi: Market Intelligence), but noting that there will be some differences due to the fact that the T-100 data includes charters, extra sections, and diversions. You may notice that there are more flights in the Schedule reports vs T-100 departures, because not all scheduled flights were operated, yet there are more markets shown in T-100 because of diversions and charters.