Perhaps Google's tried to make it run too cheaply - Kagi's one doesn't run unless you ask for it, and as a paid product it'll have different priorities.
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.
I agree with your assessment regarding the intention of the phrase. We're back at the silly arithmetic meme that hinges on not grouping terms explicitly and watching people yell at each other in the mistaken belief that there's one authoritative interpretation of an ambiguous string of symbols.
Still, the actual mistake remains. Why an extra 1/6 of the pizza? 1/3 of 1/3 is 1/9, not 1/6. That's 1/2 of 1/3.
Ah, you're right - I misunderstood jbrain's point to just be about the "relative to the original" understanding. Guess I'm no smarter than Google's AI.