The amt_rake_bb stat appears to be wildly incorrect.
For example, when I run a custom report showing total rake $ (amt_rake) and total rake in BB (amt_rake_BB) the numbers bear no resemblance to each other.
Formatting is:
Rake: format_money(amt_rake, false)
Rake in BB: format_number(amt_rake_bb,2,true,true)
From playing 25NL and 50NL tables I have figures:
25NL Total Rake $786.10 / Rake in BB 10,439.70 (36,802 hands); should be 786.10/0.50
50NL Total Rake $966.30 / Rake in BB 6,843.75 (24.623 hands); should be 966.30/1.00
This seems a pretty straightforward calculation.
FWIW I play on Stars.