Sub silent routine?

[CaesarAug]

The Manual specifies that a Submarine running in silent mode cannot naval cruise and moves only 2/3 of its normal Action Points.

Quick question: if a Sub’s normal Action Points are modded to 18, then if it should go in silent mode, its movement should be limited to 12.
Yet I notice that it can silent move 13 instead of 12, at least at game start (a modded 1939 World at War scenario). What accounts for an additional hex silent movement? Is it related to strength and/or supply levels?

Later on, after some battles and strength/supply losses, its silent movement is reduced to the expected 12. But why does it get 13 hexes before?

Thanks for any input.

[CaesarAug]

EDIT: trying out different game-start Action Points for Submarines, the game engine calculates the 2/3 hex ratio for silent running by always rounding up.