seem like the top 10 positions' weights are only displayed in 0.5 kg increments... it could just be a matter of them rounding up for Webber to get to 661 and them rounding down for Vettle to get to 661.
whereas after P10 the weights are declared to an ccuracy of 0.1kg
just something I've noticed... may not be accurate
Pos Driver Weight (kg)
1. Webber Red Bull-Renault 661.0
2. Barrichello Brawn-Mercedes 647.0
3. Button Brawn-Mercedes 644.0
4. Vettel Red Bull-Renault 661.0
5. Hamilton McLaren-Mercedes 654.5
6. Kovalainen McLaren-Mercedes 664.0
7. Sutil Force India-Mercedes 678.5
8. Massa Ferrari 673.5
9. Raikkonen Ferrari 674.0
10. Piquet Renault 676.0
11. Heidfeld BMW-Sauber 681.0*
12. Alonso Renault 668.2*
13. Nakajima Williams-Toyota 683.6*
14. Trulli Toyota 683.7*
15. Rosberg Williams-Toyota 689.6*
16. Kubica BMW-Sauber 673.5*
17. Buemi Toro Rosso-Ferrari 674.5*
18. Fisichella Force India-Mercedes 662.5*
19. Glock Toyota 662.3*
20. Bourdais Toro Rosso-Ferrari 689.5*
* declared weight