I experienced this phenomenon starkly when I started a book, and felt it was filled with obvious remarks and little novelty. It took me 50 pages to realize I had already read it.
This is the primary downside of reading without a bookmark. Any book I've read on public transportation, I would guess on average each page gets read about twice (with some chapter intros reaching double-digits). It's hard to figure out if it's really something you've read before, or if it's just a very logical continuation of what you read yesterday.
I also feel obligated to one-up you. One time in high school English class, the teacher put a sample student essay up on the projector and picked it apart in front of the class. It took me half an hour to realize that I had written the essay in question, and by that point I had concluded that I did not completely agree with it. I learned something that day about writing.
I suspect this is also a key part of your inability to determine whether or not you have read a given page before. You are reading in an environment that does not lend itself to creating notable memories. This is purely personal conjecture but I suspect that if you went and read somewhere more interesting your memory of what you are reading would magically improve.
Books I read while at home or similar tend to disappear into some sort of memory hole. Meanwhile books I have read while visiting other countries tend to be easier to recall, both in terms of the book's contents and the circumstances I was in when reading it.
Weird. I always notice from the first sentence or so if I've already read a page, so it's relatively quick to find the page I was at with binary search...
