Battery Drains when not on Low Power Mode and Battery Health has decreased by 13% in 11 months

In September, I bought a brand new iPhone 11. I transferred all my data from my iPhone 7. Even since I bought it, the battery drains faster, much much faster, when not on Low Power Mode; I have to have Low Power Mode on all the time when not charging or else it doesn’t last long enough. Low Power Mode shouldn’t damage the battery (aka reducing Battery Health) but the Battery Health was 100% in September and, 11 months later, it’s now on 87%. Why is this dropping so fast? Why does my battery drain when not on Low Power Mode? In the last 10 days, my average ‘Screen On’ time (found in settings > battery) is 4 hours with 3hr 45m ‘Screen Off’ time. When I’m at school, I’ve seen an average ‘Screen On’ time around 5 or so hours, which isn’t bad, but also isn’t the all day battery life most people get and even isn’t the 11 hours Apple has stated on the product page.

I don’t charge my phone to 100% or fully discharge it and I don’t charge it overnight. I use a 5W iPad Charger (iPad Air 2 charging brick) and an Apple Lightning cable to charge my phone. I usually don’t do any heavy work on the phone while it’s charging either.

Any help would be appreciated.