Why this grid placement system code only works on specific baseplate size?

So I made this grid placement system thingy and the block keep floating, so i decided to change the baseplate size and then it somehow works

The code:

local Player = game.Players.LocalPlayer
local plrChar = Player.Character
local mouse = Player:GetMouse()

function SetUp()
    local NewBlock =  script.Part:Clone()
    NewBlock.Parent = workspace
    NewBlock.Transparency = 0.5
    NewBlock.CanCollide = false
    return NewBlock
end

local Thing = SetUp()

function Round(Input, GridSize) 
    local X = math.floor(Input.X / GridSize + 0.5) * GridSize 
    local Y = math.floor(Input.Y / GridSize + 0.5) * GridSize 
    local Z = math.floor(Input.Z / GridSize + 0.5) * GridSize 

    return Vector3.new(X, Y, Z)

end

mouse.Button1Down:Connect(function()
    Thing.CanCollide = true
    Thing.Transparency = 0
    Thing = SetUp()
end)

game:GetService("RunService").RenderStepped:Connect(function()
    Move()
end)


function Move()
    local target = mouse.Hit.p
    local face = mouse.TargetSurface
    mouse.TargetFilter = Thing
    local norm = Vector3.FromNormalId(face)
    Thing.Position = Round(target + (norm * 2), 4)
end