Continuity mistake: When Raul is being hanged, he has a booger under his nose. In the next shot, it's gone.
Factual error: In the film, most of the characters don't wear nearly enough stage makeup. In "Think of Me", when Christine is performing in front of a packed auditorium, for example, she would have needed a lot of blush, eyeshadow, eyeliner, etc. just to look "normal" from the audience's perspective. Instead, as you can see by watching the film, she looks pale and washed out.
Audio problem: In Prima Donna, when Carlotta is on the bed that Andre and Firmin are carrying her on, the words "Prima Donna once more" are sung but Minnie Driver, Carlotta, "sings" the words after the actual words are sung.
Continuity mistake: When Meg finds Christine in the room with all of the candles after her "Think Of Me" performance, she puts her hand on Christine's shoulder, then removes it. In the next shot from behind Christine, Meg proceeds to remove her hand again.
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.