You're not... completely rightThis ISN'T a rock song, though. This is a pop song with a teensy hint of rock. Reina could sing like this in every single song if she wanted to. She chooses not to. The type of song has nothing to do with it.
The one who gets to choose how Reina should sing isn't really herself. It's Tsunku. He produces, he asks if he wants a lower tone or a higher one. Just notice that when Miki left the group, Reina suddenly started to sing in lower tones... That's because probaly Tsunku asked her to fill up Miki's shoes. Before that, she worked as a higher counterpart to the strong and low voices of Miki and Ai.
In Uta no Rakuen Reina DID get to choose how she would sing and there's where we see Reina at her best, vocally speaking.