In Puzzle 11, there are 3 tiles in the bag. There are two good bingo lines at column a and row 2, both of which cannot be blocked simultaneously. While row 2 allows more bingos, column a yields higher scoring bingos. You want to score well while leaving a tile in the bag, and keep a good scoring play next turn. DO b10 is better than o7 despite scoring 2 fewer points because it blocks column a bingos such as BROOKLET, while preserving ANRTT for TYRANT l7 (31).
If your lead were slightly smaller, you may have to block row 2, as it is a more likely bingo line. In these sort of pre-endgame situations, you must compute how often you will outrun your opponent. With a 55 point lead, you should block row 2 as you will rarely outrun any bingo. Pre-endgames often require you to calculate prospective scenarios to find the best play.