Why did Ben learn to say, "I am but a poor [foreign country] peasant boy?" Surely that'd sound more suspicious, since you don't get peasants nowadays.
Because he's 11 and that made the most sense to say.
If Granny made the international burglar thing up, why did we clearly see her in action?
She might not have a jewel thief, however, she was likely fairy spry in her younger years. That said, much of the story is that Ben doesn't know a whole lot about his grandma and so presumed she was boring. Her being in action is an example of something Ben would know and appreciate if he got to know his grandma a bit more beyond her "boring" exterior.
