Iphone 11 pro battery health

I purchased the new iPhone 11 Pro on the 6 December 2019, So the battery health was at 100% but as of today, it dropped 2%. I tried to figure out why and what did I do wrong. so what I did was when I fully charged the phone, I unplug. I didn’t play heavy games like pubg or any online game. I let it drains down to 20%. Prior to doing to maximize the performance, I had no clues how to get the best out of my battery life. Once it gets to 20%, I recharge it. I even checked the optimized battery charging to ON. So, I have no idea why it dropped 2% percent to 98% in only 3 months... I mean, this is a 3 months old iPhone.  It’s a Very poor quality as compare to samsung smartphones samsung note 9 still working at 100% battery health