Why is my IPhone X dying so fast?

So, I recently got a replacement IPhone X because I noticed the battery was draining quick (quicker than normal and also something the Apple representative suggested). I got the replacement and had restore it from iCloud and used it for a day and noticed I only got 6hrs of usually (just like my other one). Well, I looked up ways to help the battery, did everything (like auto brightness, tap to wake off, etc) and still didn’t notice a difference. So I then contacted a representative on a chat. They suggested that I restore to factory settings and monitor the battery without any apps. I could simple browse the web and play around with the applications already installed for a day. I did that and I’m still getting 6hrs. I have brightness at 25%, tap to wake off, etc. I’m starting to think it’s just because of how long I have the phone on for but I don’t know. My parents who have the IPhone 8 Plus get awhile on their phone. Like my dad had gotten 4hrs with only 72% left and he has it on for awhile, I on the other hand was at 50% within 4hrs. Anything I can do to fix this? Keep in mind this is a replacement and the guy told me it was about 2 months old. I even searched the IMEI later when I got home. Thank you all!