We use the incoming email’s Date header to date the post, while the staged user is created when the email is processed. An email dated 11 days ago but processed today could explain this mismatch (could..).
We cap future dates at the current time, but preserve older dates. So yes, the sender can supply an older date, though we’d need to check this email’s headers to confirm what happened here.
Could you check the original email’s Date and Received headers? That should help distinguish an older sender-supplied date from delayed delivery. You should be able to click the 
button to see quite a bit of details.