MacBook Pro 2015 lost battery capacity
As per this wikipedia article, 


  Batteries gradually self-discharge even if not connected and
  delivering current. Li+ rechargeable batteries [like the kind in a
  MacBook] have a self-discharge rate typically stated by manufacturers
  to be 1.5-2% per month.


Basically, even if you are not charging or discharging your Mac battery at all, it will still lose 1-2% of its capacity every month (on average). You stated your battery age as 7.2 months, and your original battery capacity as 6960 mAh. (Side note: anyone else can find out their battery capacity, original capacity, and battery age with this tool.) Assuming it discharged 1% a month while in storage for 7 months, that means your battery could have lost 487 mAh while in storage. So, it is perfectly reasonable for your 7 month old battery to have lost 350mAh. 

It is a little frustrating to have a somewhat old battery when you have purchased a new computer, but that's just how it goes. Li-ion batteries last well for about 4-5 years, so 7 months isn't too long on that scale. You will probably need to replace your Mac's battery (or get a new computer) in 2021. So, you have plenty of time to enjoy your new Mac before things even start to go downhill. But yes, that small battery discharge is normal; remember that it occurred over 7 months, not 3 days.