Ideal Pair Trade Candidate: Berkshire A and B

Here's a perfect candidate for stat arb/pair trading: Share classes A and B of Berkshire. Correlation between the two has never been lower than 95%. Still, there's some volatility between the two. On the screen shot, the top pane shows the prices separately (Class A in white, Class B in amber, at the bottom); the bottom pace shows the ratio between the two. Its oscillations show the opportunities offered by the (bounded) volatility between the two share classes.