Why is Cirium's ASM / ASK data different from that reported by the airlines?

The ASM / ASK numbers in Diio Mi and SRS Analyser are based on scheduled flights and the capacities that we understand are appropriate to the airline and the aircraft (as per their submitted flights schedules combined with our seating dataset where not provided). Our capacity numbers represent planned schedule information and what the carrier had intended to fly and are optimized for regularly scheduled commercial passenger service. 

Typically, airlines report “actuals” (flown) data, so there can be a 3-5% variance, caused by irregular operations from unscheduled (or ad hoc) flights that were not published in advance by the carrier. 

We are subjected to the schedules data sets supplied by the airline carriers but we do have business relationships with them to ensure we have everything they wish to provide to us.