I'm not sure there is a real answer. It may be that ADR and regular shares are readily changable on some markets.
One number I think I remember if that the At The Market program (part of what was just sold out back in Oct.) was described as "50,000,000 shares to be stored #$%$000,000 ADR" or something like that. It sounded like some kind of bank account system so I'd suspect there's ways to change that balance as long as you maintain enough liquidity.
Basically I expect the brokerage houses in each market keep running the arbitrages back and forth as the relationship between the markets fluctuate.