There are a couple different reasons you're seeing the Amazon Kindle sales as $0.
There is an option for people to download a sample of the book before purchasing and that is recorded in the sales number, but shows up as $0 because there was no actual purchase.
Another reason could be that Amazon had decided to put the book on sale during that period (which they do at their own discretion - we have no control over that and, even though the author has the price set at a certain number, Amazon can override that for a sale period).