My open rate statistics seem a little off

The Open Rate is calculated by dividing the number of Opens by the number of emails sent (emails sent minus those still throttled.)

If when you calculate this manually it is different than what is displaying on your stats report, it is because the stats on your report also have deducted those that unsubscribed after reading and opting-in and bounces. When a reader unsubscribes after already opting-in they are deducted from the displayed stats, but not the percentages.