2003 was primarily the first year Federer stopped S&V heavily on grass. In 2002 he was S&V off almost all first serves.
I believed he simply got better at the net and in his volleys, not going there after a stupid ball. Overall he still had more than enough net approaches, I don't think he stayed back that much as guga2120 said.
Federer net approaches
2003: 42, 38, 43, 53, 28, 39, 29