The average body has around 5 litres of blood. When you donate 470ml of that fluid, you end up with more concentrated glucose in your system, so an elevated reading would make sense. Your body is always regulating and takes around 24 hours to replace that blood.
It has also been reported that when you lose an amount of blood suddenly, the body activates its stress response. That means your cortisol and epinephrine levels increase, glycogen (stored glucose) is released and gluconeogenesis (your body creating its own glucose) can happen, all raising your blood glucose.
A research paper, in case you want a deep dive.