Quote from LaoTseuWell, so long as last 7 days actually = last week. In my mind, last week is what I did from Sunday to Saturday the week before whereas last 7 days is a sliding window. I don't mind either, I just want the header to reflect what it is.
That's exactly why I did the header text that way -- it always bugged me that what MoneyFu called "Last Week" was actually a 7 day sliding window.
I'd like to add a calendar-based This/Last Week and This/Last Month at some point, even though I personally don't see a reason to use it. I saw another addon that used some complicated math to implement "Last Week", but month would be a pain to do that way, and I think I can do it easier with os.date in any case.