WebApp and Chrome Extension are separate products which complement each other perfectly.
WebApp and Chrome Extension use different ways to collect data and to calculate it. WebApp is a cloud service. Chrome Extension works with Amazon API,so it doesn't need to store data like WebApp does. Moreover the algorithm is different :WebApp calculates est sales by the rank that it received during upgrade. And Chrome Extension gathers rank for the last 30 days and calculates average rank. So estimated sales in Chrome Extension are based on the average rank per month.