you've found your answer.
You get rakeback as a % of MGR, so, say your MGR is $100 and 27% rakeback gives you $27. Some bonuses and promos (e.g. FTP store purchases) are deducted from your MGR. So, say your MGR is $100, 27% rakeback, but you cleared $50 in bonus -> your rakeback is calculated as 0.27*(100-50) = $13.50