Info Sought On Forwarded 1855 Cover

I am not a postal historian, so the answer may be obvious, but here goes anyway.

Can anyone explain the payment on this cover? It entered the mails in Chicago on 4 Aug 1855 (per pencil notation at left), with three one-cent 1851-57 Type IV stamps paying single-weight postage under 3000 miles to Boston. From there it was forwarded to Fayette, Maine on Aug 10, with two three-cent 1851-57 stamps added and canceled with the 25-mm Boston PAID in grid. My question is: Why were two three-cent stamps applied in Boston? Was it under-weighted in Chicago and forwarded as a double-rate cover? Is there another explanation? Thanks in advance for your replies.

First, forget the pencil notation which could have been added yesterday. In fact, I highly recommend that pencil notes written on the front of covers ought to be duly erased unless put there by a famous student, such as Ashbrook or Chase. Most of the time, they serve no useful purpose and they visually detract from the cover.

Second, I agree that your analysis is likely correct. The only other possibility is if Boston considered it unpaid and charged 3c for payment + 3c for forwarding, but I see no good reason for considering it unpaid, so I like your conclusion.

Thank you, Bill. I've had this cover for some time without really looking at the rate implications (only the one-cent stamps).