This can come about bc the miners will be drawn back to BTC anytime the price is high, and then they can collect not only high fees but high payouts for mining. There will be an ebb and flow to both coins, as miners TRY to support BCH when BTC price falls, but keep getting drawn back when BTC rises via outside means.
Probably a majority of miners are mining wherever the profit is higher - and now that the BCH difficulty adjusts frequently, the mining power will follow the price ratio quite well.. Until the halfings, there won't be any major changes in hashing power.
Most of the time, over the past few weeks, BTC has been most profiltable to mine. Some miners are probably sticking to principles and mining BCH whatever it costs. Others may be minor pool members and realizing that a big amount of their pool rewards will me lost in transaction fees if withdrawing Bitcoin frequently.