It's the same for me.
Venom GT: 462 Km/h, 2.22s, PR 93.3 - reaching speed 445 km/h.
Koenigsegg ONE: 462 Km/h, 2.25s, PR 91.7 - reaching speed 429 km/h
So almost the same parameters. I would understand the differences at several km/h, but I do not get a difference of 16 km/h!
Go to Daytona Oval at Breakneck or something with your Venom and you will see....
I got 288.71 there fully upgraded.
The One:1 would take a bit longer to reach such speed, but there is no long-enough track in RR3.
The cars here mimic the real cars - although overestimating a little of everything: from performance specs (speed, accel, grip) to upgrade costs to low-speed crash noises 
Bottom line is: 2.22 s is the time for a 0-60 mph acceleration, which doesn't speak much about these cars going almost 8 times faster. If you consider 0-400 km/h, a real life Venom needs 18.1 seconds, while the One:1 would take longer than 20 sec.
0-200 mph would be found in 12.8 sec by a real Venom, and later at 14.3 sec by the One:1.
The RR3 cars must follow those trends...
Also, the One:1 has a stronger grip which, among other parameters, might influence how the car accelerates during turns.
Oh, and were your lines the same for both cars? With the Venom, there is a tendency to enter wide and slow, then step on gas while aiming at the apex (down the banking).... this also helps gain a couple mph exiting the turn, and that's what I did. 
Even more: first run with a fresh car seem slower. I just quit and retry the race, which also helps get slow bots / less traffic.
