How about this for a theory:
When Virenque returned to racing he did not dope for a couple of years because he was on small teams and couldn't afford a scandal and was still paying pennance. This would have also coincided with the hypothesized gap in organized team doping (viz Pevebage) in Europe. However, by 2003 (team) doping was once again widespread, Virenque returned to doping and, being a good responder, began to win frequently again.
When Virenque returned to racing he did not dope for a couple of years because he was on small teams and couldn't afford a scandal and was still paying pennance. This would have also coincided with the hypothesized gap in organized team doping (viz Pevebage) in Europe. However, by 2003 (team) doping was once again widespread, Virenque returned to doping and, being a good responder, began to win frequently again.