Why do I see different numbers for Commerce vs. Clickstream purchases in reporting?

These metrics are tracked from different integrations and will not always match up because the logic they use to attribute purchases (and clicks!) is not the same. 

Clickstream

Clickstream works by tagging a visitor's browser with a cookie when they click on a link in your email. All purchases made from your site by that user will be attributed to that mailing's cookie until the user clears their cookies, which usually happens when their browser is closed or if they are manually removed. 

Clickstream reporting in your account will clearly be marked as "clickstream".

Commerce

Google Campaign (Commerce) integration sends web visitor data captured through Google Analytics. This is made possible by connecting your Google Analytics account to Delivra.

Commerce reporting in your account will clearly be marked as "commerce". For instance, the pie chart in the Mailing Overview report is pulling in Commerce data from Google.