iPad mini 6 battery

I just got my iPad mini 6 and in the first 5 days I always played Genshin Impact in 30fps with my Apple Pencil attached (average playing time 3 hours). My average iPad screen time was around 8 hours with lower power mode and I charged it once a day.

Starting from day 6 the battery drained faster when I play the game even though I use EXACTLY the same setting as the first 5 days. Once I played the game for 2 hours and the battery just drained 25% with a slightly warm temperature. But now it drained 18% by playing for 1 hour 9 minutes with a higher temperature than usual. I tested it again the next 2 days and the result was quite similar.

I’m confused is that how it’s supposed to be? Then why in the first few days it didn’t drain that quickly even with the same setting/condition?