you have at the main weapon slot (default on key 3) always 2 weapons....the axis and the allied one.
For example if you are a Medic if you shot 30 bullets from mp40 then you could press 3 again and you switcht to thompson with another 30 bullets.
But with sten you have only one weapon on 3 so if you fighting someone with two weapons at slot 3 he has the double amount of ammo befor he has to reload.
Hope its better to understand now .