Because they spent an entire math class period earlier that week explaining to the students what "reasonableness" was going to mean on their next math test, and in the context of (I'm guessing 3rd or 4th grade) arithmetic the important thing they're trying to teach is that 5/6 is a larger fraction than 4/6. I agree that the question could be worded better (change the last two sentences to "Marty says he ate more pizza. Is this possible?") but I strongly suspect that the missing context from their class - or maybe even at the beginning of the test - explains enough to get the answer the teacher was looking for here.

Yes, one kid starting with a larger pizza changes the situation, but fundamentally that's an algebra question, not a "learning fractions" question.