Subject: Re: A vs B
Indeed, currently about 1500.9 : 1
(comparing midpoints of bid/ask)

Three possible explanations spring to mind
(a) Berkshire is not buying at these levels, so there is not an "extra" demand for A shares
(b) Traders think Berkshire is not buying at these levels, so they are backing off on what they will trade A shares for
(c) Random fluctuation for reasons no-one will ever know.

Jim