AC looks at the historical, actual block times on a city pair. So the block times are realistic, but they are using the 50-60th percentile. So lots of flights will arrive early also. Delta must have a very high percentage of flights arriving early with their methodology.

There is also some apples and oranges between airlines when it comes to establishing the out and in times. Park brake release establishes the out time for the majority of carriers, but procedures differ for when that is allowed to happen.
At AC, everything has to be ready AND pushback clearance from ATC obtained before the brake is allowed to be released. So a flight could be ready to depart on sked, and be held at the gate for a few minutes due to traffic behind on the ramp and this will show up as a late departure. Other airlines have different procedures that aren’t as strict as AC and will show an on-time departure with my example above.