The thing is that "Top 10 products" page of the admin area bases its results on data from xcart_orders, xcart_order_details tables. It just checks available (!) orders and check how many products were bought during certain period of time.
Bestsellers module gathers statistics from xcart_products table. The table contains sales_stats and views_stats fields. Each time a customer sees product details page, cart increments amount of view_stats field. And if the product was sold (order status == Process), cart updates sales_stats value.
Date in xcart_products table does not depend on amount of available orders (possibly some orders were already deleted) but on overal statistics during life time of the product. Maybe you were testing this cart and were purchasing one product more frequently and then deleted test orders, but xcart_products table saved overall sales and views statistics, thus this product is still shown on first place.
If you would like to reset this statistics, please invoke the following SQL query in Patch interface or SQL manager phpMyAdmin of your store:
update xcart_products set sales_stats = 0, views_stats=0;