From advanced motorists
"WHEN STATIONARY
When stopped, with the handbrake in the off position and with a drive gear engaged, there will be a tendency for the vehicle to creep forward. The amount of creep will depend on the particular gearbox design. It will also vary with the engine idle and choke settings. This is noticeable, for example, just after starting when the automatic choke is in operation"
So it doesn't always and it varies. My current auto hardly moves at all.