Most publishers pay quarterly, so you are only getting your statements four times a year, and there is a one quarter delay for third party sales, so it can take six months to see how your book really performed.
Let's say your book was released on August 15th. The quarter ends September 30th and you get paid in October.
Your first royalty statement will reflect sales made on the publisher's website for a month and a half (August 15- September 30).
The next quarter ends December 31st and your next royalty statement, the one that reflects how your sales were on Amazon, Bookstrand, All Romance E-books and many other third party distributors, won't come to you until January. It's going to take from August to January to understand how that book sold.
Remember the Amazon ranking post? During that August to January span, you are hopefully contracting and publishing more stories. Your theoretical January Royalty statement ought to include the publisher website royalties from a few more books that you won't see the big picture on until April.
Having that many time-delayed balls in the air makes it very difficult to develop a cohesive picture of what your audience responds to, and how you as an author are progressing toward your goals.
This is where Excel is your friend. You really should have some sort of bookkeeping document set up to track your royalties, for budgeting and tax purposes if nothing else, but there is a lot more you can make Excel do for you.
I'm going to show off my fancy "Royalties Over Time (All Books) Graph" for you now:
"January Thaw" and "One Night Contract" have similar themes and plot elements, and they are my two most successful books. This established for me that readers respond to that type of story from me.
There is also this graph:
But this graph tells me that stories about college age men exploring their sexuality works for me, and that maybe this is the sort of story I should be working on for free reads or anthologies.
There other correlations I can look into from the first graph. The Purple and Light Blue peaks from the first graph are "Summoning Sin" and "Sun Goes Down", very different books and story types, but both paranormal. They did similarly, though "Sun Goes Down" did a little better. Was that because it was my second paranormal release, (with Evernight), is it because of the vampires? Is it because it's really one my favorite stories I've written and I was super into it? I would need more data to figure that out. But with the data I have I can create this graph
You also can use your royalty information to answer questions you have for yourself about your writing such as "Should I concentrate more on M/M or M/F?
The Pie Chart has spoken. M/F may not be my strong suit.
And you can always make graphs just to make yourself feel better. A lot of us crank out book after book in a vacuum, with huge stretches of time between validations like a great review or a royalty statement that pays for anything significant. In times of doubt, I can pull up my royalties work book and ask:
"Am I really getting any better, oh wise and powerful Excel Spread Sheet?"