this was on the browser version
I went to the menu and searched for mario mario and click search and then open page then closed the book and did it again and it was a different book!
By "did it again" you mean you opened the menu and clicked search again?
uh huh yes
The same piece of text can be found in multiple places in the library. The search menu randomly picks one of these locations each time you hit search.
You can verify that the text was actually in the book that you searched for before you searched for it by remembering the exact location of the page (hex, wall, shelf, book and page number). You can go to this location at any point in time, in the browser version or the offline version on any machine and will find the text that you searched for.