Cuz GS not ingame value, this system must be based on avg ilvl.
From what I have read the characters ilvl is calculated by just adding up the ilvl of all your gear and then divide by the number of slots it takes up.
Which would mean Smourne increases your characters ilvl by the same amount like a ring or bracers of similiar ilvl.
That would mean the characters ilvl doesn't reflect your stats since a weapon is a much bigger improvement than a ring.