In general, these two reports are not meant to be compared. The Sales report may include no-auth payment methods, while the Settled Transaction report does not.
The Sales report also records most transactions on the sale date, while the Settled Transactions report shows transactions on the day that they settle. The sale and settle dates are often different.
Also note: the Settled Transaction report will include any tips that were processed on a credit card. The Sales Report won't note the tips.
For more accurate results, you should instead compare the Settled Transactions report with your bank statement. Keep in mind that transactions are deposited into your bank account 2 to 3 days after they are settled.