Lab 4.2 - not sure why my answer is wrong

Kind of surprised by the answer for lab 4.2. I was able to get a smaller index to service the query and the answer was wrong even though the execution stats were the same. actually my execution time was faster.

this is kind of misleading in the question

“but we’re looking for an index as small as possible that services this command”

I thought maybe b/c I was actually using mongo 3.6.8 and not 3.4 however I spun up a 3.4 instance and the aggregation still uses my index which is smaller to service the query