Charge going down while charging

Hi,

I just bought a brand new apple watch series 4.

I've put it on the charger and set it up using my iphone.

The iphone tells me that the watch is charging (lightning bolt), but the charge seems not to be going up, and even went down a bit then up again - it's going up and down between 19% and 23%.

Is this because the watch is downloading a lot of stuff as it's just brand new or should I be worried? (look at the watch app it seems to be downloading music, app updates, os updates and so on).

I was also a bit surprised it was delivered with only 20% battery charge as I thought they were normally supposed to be around 50% for transport.

edit: it's plugged into the mains and not a computer before you ask :)