Even if you are not a writer or a critic, reading a book a second time might shed a different light on it.
Most people are not that much in the literary aspect of their reading. They just love to read a story and get enjoyed by it.
That is why a lot of literary style elements in modern novels are not that important anymore for the average public. Keep in mind that if you are going to write in complicated, compounded sentences many readers might easily throw the book aside, due to impeding the reading speed.
Not all readers are academics that understand complex grammar and not all are willing to take the effort to analyze sentences to enjoy the beauty of the language.
That is why many writers today write in a less complicated style, especially when you want to write for a general public.
Still it is always rewarding to read a book twice. Why? First time for the story and second time for the style and language.
Even if the book is written in a simple language, it might be nice to study it. Also are books written work that are edited before publication. At least: all published books are. Self-published authors might have to do it all themselves and might leave some minor mistakes in their texts.
Even if a writer had a college degree, it might not be in language. That is why most are not capable of writing a full manuscript without any errors. Ask publishers: they seldom get work that does not need some polishing.
Professional editors might overlook things as well. Less chance to it than when the writer edits his or her own work, but still some errors might be overlooked, surely in long content like a book.
Concentration goes in fluctuations, so it might go down and have peaks at other moments. All human beings have this, not only writers and editors. As a reader, you encounter the same problem.
So yes, you may over read things, due to the concentration lapse. When your read again you might not miss it.
This does not mean searching for errors, but finding nice style elements, or rediscovering twists in the story line you did not look at in the first reading.
Also may you discover that there are great writers, but none is perfect. You might also encounter some style errors in published and awarded work.
Yes, most published work might be error free when it comes to spelling and grammar, but it does not guarantee that all written there is in a correct style.
Some writer do tend to use overly wordy phrases. Others might use a lot "there was", which is considered poor use of language and still those things get published.
If you want to be a writer, be sure your work might not be perfect as well. But by noticing the style of another, you may discover your own and learn to read and write better yourself.