Aerodynamics. The Zonda is built for maximum speed on the track, which means lots of downforce, and lots of drag. Straight line speed is not the goal. The Jaguar was optimized for top speed, which means that they kept the drag as low as possible (but there's hardly any downforce)
It's like this article, where Edmunds took the Viper ACR to Bonneville:
2008 Dodge Viper ACR: Flat Out at Bonneville
The ACR is not designed for top speed, so it didn't even hit 200, despite having 600 hp. Design flaw? No - it's faster than a ZR1, a Porsche turbo, or a GT-R on the Nurburgring.
That's why people care about 'ring times, not top speed. Personally, I'd rather have a Caterham CSR 260 than either of those cars, and I'm not even sure it can get past 145...