The talk time is highly dependent on the cellular network environment such as the distance to the closest GSM cell tower. Moving fast while talking (as in a vehicle) also negatively affects battery life.
Manufacturers measure talk time in controlled conditions and the quoted numbers are rarely reached in real-life scenarios. These numbers are best used as reference when comparing phones from the same manufacturer because battery life measurement methodology will probably vary from manufacturer to manufacturer.
Quite often you will find the Talk time field in the specification pages of new phones empty as the manufacturers sometimes fail to disclose the info at the time of the official announcement. Later on, when the handset hits the market, the figures are usually revealed and accordingly, we take the care to add them to our database.