Probably should've just asked Wolfram Alpha
Yes, and the Google AI response is correct (and quite clear) in what it says. edit: Thanks Batman. I mean that Google's understanding of the question is logical (although still the maths is wrong as you say (now I've re-read you)) and its answer explained the angle it was answering from.
However, I think the reasonable assumption for the intention behind the question is relative to a whole. I had third of a pizza, and now I have an extra sixth of a pizza. It's subtle, but that's the kind of thing AI falls down on.