Easter egg: On the Bonus Features DVD, disc 2, on the main menu just under "The Show," there is a small curly square of decoration that illuminates when you click it. It shows the trailer of the film.
Easter egg: On the Bonus Features DVD, disc 2, in "Under the Movie" there are two Easter eggs. One is located on the bottom left hand side, click on the small curly square of decoration that illuminates - the Phantom sings a solo "Learn to be Lonely." The second is above this one (top left) which consists of Emmy Rossums' screen test. (Region 2 DVD)
Easter egg: On the Special Features Disc, in "The Movie" menu under "Features," at the bottom, on the left, click on the curly part of the decoration so it's highlighted. It shows a cast/crew sing-a-long for the song "The Phantom of the Opera."
Answer: I like to think that he would always place a rose there, possibly every week or so, and they never had the ring on them. And he knew When Raoul would be there, so he placed them when he knew he would be alone with her. As for the two years later, since the phantom "knew" when Raoul came... he especially put that ring there. Just to let Raoul know about his love for Christine and the fact he is still living. He knew Raoul would visit then, and knew what to do. (This is only a theory I have proposed. As for a genuine answer... I am not aware of one.) Hope this maybe helped.