One thing that all grandmas have in common is that they lack a filter. They say whatever’s on their mind without thinking of the consequences, perhaps because they know they can get away with anything. They’re old enough not to care what people think about them, and you have to respect them, either way.
It’s a good thing this girl didn’t understand her grandma’s response at the time, and now when she’s older, she probably finds it funny. And hey, we don’t know if her grandmother was trying to be offensive, or if she just answered her question very literally. Either way, she’s probably not wrong.