Why are there tiny traces/marks on the side of the scale?

Actually this is normal. It' s the production merging joint places. The shell is made of plastic and it ' s in liquid status at first.
Then we inject them into the module and the liquid merges to the end, the liquid may have some pattern when they merge.
The marks you circled were made naturally and inevitable.  We already make the joint place to the less important places.
It' s not that obvious and less noticeable. 
Let me know if you still have any question please.