UEFN inherits its grid-snapping values, known as Unreal Units (UU), from Unreal Engine (UE) for all actors. Inside UE, 1 UU is equal to 1 centimeter (CM). This means smaller snap values cause the actor to snap in smaller increments, or alternatively, larger snap values cause actors to snap in larger increments. This is the opposite of how grid snapping works in Fortnite, where the smaller the grid-snapping value, the larger the incremental snap.
However, Building Actors do use the Fortnite grid snapping values. You can enable and disable Fortnite grid-snapping properties in World Settings by toggling Editor Cell Snap on and off for Building Actors. Changing your grid snapping levels does not affect non-Building Actors.