Originally Posted by Nightflyer787

There are also some subtle and important differences in procedures between airlines that affect the recorded departure, or “out” time. At most airlines, when doors are closed and ready for pushback, the parking brake is released to establish an out time. If ramp control delays the pushback, these operators will still recycle the parking brake to establish the out time and absorb the subsequent delay in the block time.
AC does not permit the brake to be released until pushback clearance is received, so there are many, many short delays due to apron control delaying pushback due to apron congestion etc that show up in AC statistics as a late departure whereas other airlines would be on time in the same situation.

Additionally, YYZ has recently implemented full A-CDM after a couple years of trial and loose compliance. (Airport collaborative decision making…google for more info). This is causing many short delays at pushback to, in the eyes of the pilots, satisfy some invisible bureaucracy and accomplish nothing other than delay the departure.