Maybe iBooks is more efficient? And, most importantly, this was back on iOS 9. I wouldn’t be anywhere near those 21 hours now on iOS 12.
As I mentioned before, I got 10-12 hours max on the Pro 9.7 on iOS 9 when it was brand new. After a year of super heavy use and battery abuse (drained to 0% quite often), that was down to 8-10 hours (still on iOS 9).
As for iBooks being more efficient, I doubt it makes that big a difference that it gives double the battery life vs Marvin.