Harrington on Hold'em VII

[n8dawg]

i have a question about a comment that he makes that confused me, if you read the book or have the book page 240-241 problem 10-1. On page 241 it says that the BB didnt make any errors since it was possible that he had the best hand. However he only had an M of 2.4 so wouldnt the proper play be to go all-in with his hand anyway. once BB calls your bet isnt he pretty much commited to the pot anyhow? so what i dont understand is, why does harrington say is the BB didnt make a mistake.