OY VEY !!!
Imagine this:
You have a 10 inch tall glass (this is your engine block) filled with Coke up to a height of 4 inches (this is the MAX mark on your dipstick).
However your straw (this is your dipstick) is only 8 inches long.
When you fully insert the straw, you have access to only 2 inches of Coke which you can drink (measure), even though there are still 2 inches of Coke left in your glass. But, you cannot reach/drink/measure those 2 inches left.
The gist is that your engine oil dipstick is not designed to reach all the way to the bottom of the oil pan. This way, when you pull a dipstick out, and it is dry, that does not mean you have no oil left in the engine/oil pan. Just that you do not have ENOUGH oil for the dipstick to measure.