Do we always remember what we read?

I've been reading the sequel to the Relic by Douglas Preston and Lincoln Child

Last night, I got to the last 1/16th of the book and realized I had read it before!
But why didn't I realize it some 424 pages before!!!!! Didn't I read it thoroughly the last time?
Did I toss out the memory of the book to replace with something else--maybe a better book?
I hope not. This book has been entertaining in its own right. If I replaced the memory with anything, I hope it was with a good kiss--the kind that makes my toes curl.