Media Markt Sold Me an iPhone 15 Plus With Battery Health Dropping in 2 Months

I bought an iPhone 15 Plus from Media Markt on September 24, and in under two months, the battery health decreased to 99%. This raises some concerns, especially since a friend who purchased an iPhone 13 three months prior to my purchase still enjoys 100% battery health, while an iPhone 14 Plus that's a year old is at 96%. I consistently use the original cable and socket for charging and refrain from using my phone while it charges, so I’m puzzled as to why my device’s battery health is declining so rapidly. I anticipated a higher level of quality and performance from a brand-new device. I think this is a matter that requires attention, and I would appreciate Media Markt's help in finding a solution.