The piano he used from 2002 to 2009 was a digital piano inside the case. Since the piano was re-introduced in 2010 he has started using his traditional Knight piano. I would assume it's something to do with technology now allowing him to use his Knight to produce digital sounds for songs like New whereas he needed the digital piano before. We've also seen him use the grand piano to produce a harpsichord sound. Sorcery!