why not use $Ap - p$ instead of $p - Ap$? I am watching this video about the Leontief input-output model: https://youtu.be/hlaBURtSDO8?t=12m26s

One thing I don't understand is: why not use $Ap - p$ instead of $p - Ap$? It seems to make better sense to measure net production by output - input. Maybe I am missing something?
$p$ represents total production.

$Ap$ represents the intermediate goods and services used in production, i.e. intermediate consumption.

So $p-Ap=(I-A)p$ represents net production, i.e. output minus input.