I’ve spent several hours collating stats only to come to the conclusion that I cannot compare like with like due to the many changes Amazon have made in the last few years.
In 2011 books were sold and we authors received a royalty. In December of that year, the first change appeared - books were either sold or borrowed. By the end of June 2014 we had the introduction of KU/KOLL units. I understood that KU stood for Kindle Unlimited, but I was never sure what KOLL stood for. (Now I know it is Kindle Owners Lending Library)
By July 2015 we saw the introduction of “Net Units Sold or KENP Eead.” Otherwise explained as: Net Units Sold or Kindle Edition Normalized Pages (KENP) Read. According to Amazon people bought books but never read them, so this was an attempt to pay only on pages read in an effort, so they said, to weed out the dross that was self-published.
When KU came in, KENP made some kind of sense. Members of KU didn't buy the book, but "borrowed" it, so no royalty accrued to authors. KENP means that for every page read the author is paid a tiny amount, something like 0.0016 of a penny. On a 400 page book that would amount to 64p. It was as much, sometimes more, than the original royalty payment on low-priced fiction.
So far, KENP Read is still holding. The only way I can see that I might be able to tell if I am doing better or worse is by tracking the money. Is the money that comes to me via KENP pages read better, worse or equivalent to my payments back in 2011? Who knows? Looks like another few hours doing stats. Even then, other things have moved and changed since those days, so I won’t ever be certain whatever the stats tell me.