What sort of weight variation is normal?

Yes, these fluctuations can certainly occur. Water balance, digestion etc.

It is best to weigh yourself at the same time and under the same conditions. For example, in the morning after waking up, after you have been to the toilet and have not yet had anything to drink.

Hi - I make a point of always weighing at the same time and, let's say, at the same point in my daily routine. :-)

That's why I was surprised at the size of the fluctuations. 2kg is quite a lot.

Is there a difference in how the scale is stored? If the scale is stored vertically, you should tare the scale before taking the next measurement. The steps to do so can be found here (I bring this up because 2kg is roughly the amount the scale is usually off for the first measurement after being stored on it's side)

That's interesting. I do store the scale on its side so this could it. Thanks 

Scale not accounting for its own weight? - Index S2 Smart Scale - Health & Wellness - Garmin Forums