why does it do this?

[simple igloo]

local Players = game:GetService("Players")

Players.PlayerAdded:Connect(function(player)
player.CharacterAdded:Connect(function(char)

    local rightArm = char:WaitForChild("Right Arm")

    local container = game.ReplicatedStorage:WaitForChild("Container"):Clone()
    container.Parent = char

    local handPos = container:WaitForChild("HandPos")

    handPos.CFrame = rightArm.CFrame * CFrame.new(0, -1, -1)

    local weld = Instance.new("Weld")
    weld.Part0 = handPos
    weld.Part1 = rightArm
    weld.C0 = CFrame.new(0,0,0)
    weld.C1 = CFrame.new(0,0,0)
    weld.Parent = handPos
end)

end)

[jaunty monolith]

Because when u weld something you move it to the position of the centre part

Place the cframe line after the weld 👍

[simple igloo]

still same result