Explain why a %10 increase followed by a 10% decrease is less than the original amount if the original amount was positive.

Because a percentage increase or decrease is always a percentage
of the number you're operating on.

If you first increase by 10% and then decrease by 10%,  then the 
decrease is based on a larger number than the increase was, and
you lose more than you gained.

10% increase of $100 is $10 gained, giving you $110.
The 10% decrease is then based on $110, so you lose more than
you gained, and end up with only $99 .

Furthermore ... the Universe is a cruel place.  If you were to decrease
first and increase last, the decrease would still operate on the larger
number, and you would still lose.