My guess as the potential reason is the difference in engine placement and weight.
The 35is engine is heavier, larger and placed more in front of the front axle. The 28i engine is smaller, lighter and is placed more behind the front axle. Is the rationale that the 28i is dynamically better so it is classed higher?
Can’t ignore the power differences though. 0-60 in 5.6 in the Z428i vs 4.6 in the Z435is.