Celeste, I think a big difference is that when a person is playing a solo piano piece, there is room for things like rubato, gentle nuances, and even a "liberty" or two in interpretation. In a concerto, not during a piano cadenza but where piano and orchestra are playing together, things are more governed by the conductor's baton, the need to keep things synchronized, and the the struggle sometimes for the piano to be heard. I think it was Horowitz who used to joke that playing a concerto was simply having to play in time. But in a way, I don't think he was joking. He knew all too well that integrating with the orchestra meant sacrificing some of the individuality of the performance, where the purity of sound, as you put it, becomes a concerted sound and, by necessity, strictly in time too.