When half of BTC holders will have exchanged their BTC for BCH, then it will be a 50/50 split. Until then, you need fresh money to add value to the new chain. I think this is quite intuitive from a macroscopic, detached point of view.
Sounds like you are saying that there is currently something other than the 50/50 split.
Can you elaborate more on "half of BTC holders will have exchanged their BTC for BCH"?