If you're paying close attention to your earnings every day, you will likely notice that sometimes you'll have a sale but no earnings or you'll have earnings but no sale. This is due to how Amazon reports earnings.
Once a customer buys your book, you will immediately see it show up. However, earnings don't show up until the payment from the customer fully clears. This can sometimes take a day or more, so it results in a sale with no earnings.
If you see earnings but no sale, it's likely because the earnings from a previous sale just cleared.