what's the difference between aggregation type "max" and "max_pop"

I notice that there some aggregation type pairs like "total"/"total_pop" and "max"/"max_pop".

Eg.

Aggregation Type	Description
max	The measure is aggregated by taking the maximum of the values of all child cells at the base intersection.
max_pop	The measure is aggregated by taking the maximum of the values of all populated child cells at the base intersection.

Questions:

1) To my understanding, "all child cells" includ the one with default value, but "all populated child cells" doesn't. Am I correct?

2) Does the different type come up from consideration of calculation performance?

Thank you.

BR/Phil