Bernard, I wonder if it's the way the data is interpreted. Estes did exhaustive tests on all of their motors back in the day. They may have had a way to estimate the point at which all of the black powder fuel was consumed from their data, or it could have been as simple as knowing the burn rate of the fuel and the configuration of the grain in the motor.

Initiator's altimeter is probably reporting end of burn time as the point at which acceleration goes to zero (in other words, roughly when all propellant gases have escaped through the nozzle). These things aren't necessarily the same thing, and it's reasonable that Initiator's estimate of burn time would be higher.