Subject: Re: Book Value & Repurchase History
The repurchase algorithm is likely more than just "anything below xBook", it likely looks like this.

1) Is there excess cash? --> Yes
2) Are there any attractive investments --> No
3) Is BRK cheap --> Not yet

Also, "Is BRK cheap" is probably not "is it below xBook". Buffett told us how to value Berkshire:

"I believe Berkshire’s intrinsic value can be approximated by summing the values of our four asset-laden groves and then subtracting an appropriate amount for taxes eventually payable on the sale of marketable securities."