Perhaps the As are being bought at market price when a large block is offered for sale.

Certainly possible, but that is presumably a pretty rare thing.

Since spring 2022, for months that both A and B shares have been purchased, the average ratio of average price paid has been 1531.4
(The 1540 I suggested emphasized recent high points, and included the notion that there seems to have been an upwards trend. In November the ratio was 1544:1)

1530 is of course the 2% premium that Mr Buffett suggested long ago.

The simplest indication of the recent stronger preference for A shares is that that's what they've been buying.
In the 23 months from May 2020 to March 2022, both A and B shares were bought every month.
But then in the 22 months to March 2024, A shares were bought every month but B shares in only 9 of the months.

I'm sure they'd pay more now, but FWIW the highest definitely known "average paid in a month" for B shares to date remains the $357.22 from last September.

Jim