The schedule doses (the directions on the package) make an attempt to follow the doses you would get if you calculated a mg/kg dose for an average sized child. However, since the dropper is only marked with a half and full mL, the schedule is necessarily a series of steps.

By following the schedule, a child will be under-dosed just before a step. For example, at point A the child is almost 4 months old and still getting a half mL dose. At 4 months the average child weighs 6.32 kg (14.93 pounds). Canadian acetaminophen is 80 mg/mL, so a half mL is 40 mg and the average child using it would receive a dose of 40mg/6.32 kg, or 6.3 mg/kg. The next day (point B), when the child is 4 months old and goes to the next step in the schedule, the dose is doubled to 12.7 mg/kg. (I do not know the target range in Canada, in the USA it is 10-15 mg/kg.)

Dosing exactly by body weight (at 11 mg/kg, or 5 mg/lb) yields an exact dose for each child. At 4 months, the average 6.32 kg child would get 6.32 kg X 11 mg/kg, or an exact dose of 69.5 mg acetaminophen.

Dose response graph