I don't have an answer yet!
The way I really understand it, is that its estimating by the highest place value, not a very close estimation, but still around the number.
It's to do with how numbers compare with each other in size, although very approximatly. For example, 36243 is an order of magnitude higher than 5381. When written in standard form the first would be...
add projected sales in units to desired ending inventory and subtract beginning inventory
Wet your index finger and put it in the air. Think of a number and double it.