TweenService inconsistent tweening speed

So basically I noticed that if you create and play a tween right at the start of the server, the time it takes for the tween to complete is significantly (up to three seconds!) shorter for some reason.
Code used:

local TweenService = game:GetService("TweenService")
local TestValue = script.Value

local TweenLength = 10
local TweenInfo1 = TweenInfo.new(TweenLength, Enum.EasingStyle.Linear)
local Tween = TweenService:Create(TestValue, TweenInfo1, {Value = TestValue.Value + 10})

print(string.format("%.1f", os.clock()))
local DeltaTime = os.clock()
Tween:Play()
Tween.Completed:Wait()
print(os.clock() - DeltaTime, string.format("%.1f", os.clock()))

Result:
grafik

For some reason this doesn’t happen if you launch the server through Studio and the result is pretty much as expected:

If anyone knows why this inconsistency occurs I’d be delighted to know why!