Why doesn’t sorting by Bestsellers correspond to my actual data in sales reports?

Please check the extension settings highlighted on the screenshot below:

If you've enabled the option Use Index, you need to run re-index after changing the settings.

As for the reports, most likely your reports include data from completed orders for the bestsellers report while you haven’t excluded any other order states from Bestsellers.

Collect the information on how your bestsellers affect the sales level with the Advanced Reports module for Magento.