Incoming Max Limit Exceeded, but not really exceed.

Incoming Max Limit Exceeded, but not really exceed.

I was just presented with an interesting situation where the attachment (PDF document) size was only 15.6MB ( or if we round up, 16MB) yet the email was not fetched due to exceeding the 20MB limit. I realize there are other parts of an email (headers, body text, etc) but I'm finding difficult to understand that the remainder of the email equaled 4MB. 

Please advise.

Thanks for your assistance.