Simple...the use of reduced thrust
Most operators are approved to use reduce thust on departures, some as high as a 20% reduction from maximum thrust, with governmental, manufacture and operator approvals. This is all very safe and a good way to operate. Taking an engine to 100% expose the operator to a statically high expose to engine failure than a reduced thrust. Do you accelerate you car to max rpm before every shift? Of course not, I hope.
So timing the departure roll could be very missleading.
Hope this helps.