Can someone explain why this placement must divide and stuff instead of just addition?

local Player = game.Players.LocalPlayer
local Mouse = Player:GetMouse()


local PosX
local PosY
local PosZ

local Model = game.ReplicatedStorage.Crate:Clone()
Model.Parent = workspace

local GridSize = 5

local function snap()
	PosX = math.floor(Mouse.Hit.X / GridSize + 0.5) * GridSize
	PosY = Model.PrimaryPart.Position.y
	PosZ = math.floor(Mouse.Hit.Z / GridSize + 0.5) * GridSize
end

local function Movement()
	Mouse.TargetFilter = Model
	
	snap()
	
	Model:SetPrimaryPartCFrame(CFrame.new(PosX, PosY, PosZ))
end

Mouse.Move:Connect(Movement)

At the snap function, all it does is add 5 to the position. Why does adding + 5 not have the same effect?

math.floor(x) rounds things down

math.floor(0.9)
math.floor(1.0)
math.floor(1.1)

gives 0, 1, 1

math.floor(x + 0.5) rounds things normally

math.floor(0.9 + 0.5) = math.floor(1.4) = 1
math.floor(1.0 + 0.5) = math.floor(1.5) = 1
math.floor(1.1 + 0.5) = math.floor(1.6) = 1
math.floor(1.4 + 0.5) = math.floor(1.9) = 1
math.floor(1.5 + 0.5) = math.floor(2.0) = 2

math.floor(x / y + 0.5) * y rounds to the nearest y.

math.floor(6.73 / 2 + 0.5) * 2 = math.floor(3.865) * 2 = 3 * 2 = 6
math.floor(6.73 / 5 + 0.5) * 5 = 5
math.floor(6.73 / 10 + 0.5) * 10 = 10
math.floor(6.73 / 20 + 0.5) * 20 = 0

In snap, x is Mouse.Hit.X and Mouse.Hit.Z and y is GridSize.

local function snap()
    PosX = math.floor(Mouse.Hit.X / GridSize + 0.5) * GridSize
    PosY = Model.PrimaryPart.Position.y
    PosZ = math.floor(Mouse.Hit.Z / GridSize + 0.5) * GridSize
end

snap is using this to round X and Z to be multiples of GridSize.