The third week stall that is so common signals a change in your weight loss character, from the initial loss which comes primarily from glycogen (short term reserves of stored carbohydrate) and the associated water weight to the longer term draw from our fat reserves once the glycogen has been depleted. This can take some time to shift gears (and sometimes very little.)
I, too, never had a third week stall, but there was certainly a slow down in loss rate right at the three week mark (fat burns more slowly than glycogen/carbohydrate, on the order of the classic 3500 calories per pound vs. around 2000 calories per pound.) My only real stall (a week without loss, by my definition) was at about four months for a week, when I was travelling, so it was possibly a result of increased sodium intake from eating out more (I usually would gain 2-4 lb on those trips, which would dissipate within a week - classic water weight.)
Why didn't we stall and others did? It might be random chance, or it might be that our metabolisms are still fairly robust, or maybe diet is not a low in carbohydrate as many maintain, so the glycogen reserves were able to more quickly return to a functional level.
Whatever the cause, enjoy the ride!