In Chapter 11 of The Westing Game, why did Mr. Hoo laugh when Grace suggests that her husband is doing his bookkeeping?

Unwittingly, Grace Wexler has just made a joke that she is
unaware of. What she doesn't know and what her partner in the game does know, is that
her husband actually likes to go to the bookies and gambles. Thus, when she innocently
says that Turtle may be helping her father with his bookkeeping, she is unknowingly
making a joke that is understood and appreciated by Mr. Hoo. Of course, the idea of her
husband corrupting little Turtle and leading her astray through his penchant for
gambling makes her ignorance even more hilarious. This is of course one of many such
jokes that echo through the pages of this novel as we come to know a set of characters
better than they know themselves and each other.