It means BTC is mining on average more blocks than BCH and if this state would remain for the next ~600 days, BTC would surpass number of blocks BCH has mined before the halving.
But it's unlikely to happen, as pools would need to deliberately keep BTC mining diff low, and then benefit from it with increased hashrate.
According to my 90 day data sample, BTC will halve 7 days after BCH.
