Why the Same Android App Works Differently on Different Phones

I have two Android phones sitting on my desk right now, a Google Pixel 9 and a OnePlus Open. A few days ago I noticed that Discord notifications showed up instantly on my Pixel but arriving late on the OnePlus. It’s the same app with the same version number, using the same Wi-Fi network and the same Discord account. I figured it was a Discord issue, but it turns out it’s not. Discord is just one of many apps that behave differently across Android phones, and there are several reasons for that.