Based on your screen shot Fortnite would run for over 4 hours, 7% in 21 minutes is just over 20% an hour.
Also you are comparing 2 devices with completely different hardware and operating systems, depending which iPad you have the battery is 3 or 4 times bigger than your phone. The game may be better optimized and draw less power on an iPad. Point is its not an equal comparison.
And Fortnite is one of the most demanding games on mobile right now, it's a battery hog because it's pushing the hardware to its limits. Most Android phones aren't powerful enough to run Fortnite. I've played it on my S7 and Note 9, probably drains the battery faster than anything I've ever done.