semplicemente perchè il mese parte da 0 fino ad 11.

basterebbe consultare la guida ogni tanto:

Availability

Flash Player 5.

Usage

myDate.getMonth()

Parameters

None.

Returns

An integer.

Description

Method; returns the month (0 for January, 1 for February, and so on) of the specified Date object, according to local time. Local time is determined by the operating system on which the Flash Player is running.