"I have a real 5-year IV growth (5-groves) of 4.8% CAGR."

I am getting book value increase of 11.5% CAGR for the last 5-years. ChatGPT tells me that the average inflation in the last 5 years has been 4.5%. Hence, the real increase in book value was likely around 7%.

I'll change the real 5-year IV growth (5-groves) to 5.4% CAGR. I had bad inflation data.

Using the same inflation data, I get the same as you for book value growth: real 5-year book value per share growth 6.9% CAGR

What might explain this discrepancy?

All that cash with a close on zero real return? Cash + fixed went from 37% of book to 54% of book over those five years.

Anyone have yearly ROE numbers for Berkshire?

Separately, if real IV growth has indeed been only ~4.8% [changed to 5.4%] annually over the past five years, does a price-to-book multiple of 1.5–1.6x seem justified?

No, that 5-groves calculation I'm using now has a price to book of 1.44. More of the firms assets are being put into things that should be valued at 1x book, or possibly a little less.

Berkshire has repurchased shares at levels just below ~1.5x book, which implies that management views intrinsic value as at least 1.5–1.6x book (given their mandate to buy back stock only below their conservatively calculated estimates of IV).

They haven't repurchased many at those levels, and they were nearly all A shares. The vast bulk of repurchases have been at 1.4 or less.