Could it be that they are referring to two different periods? Maybe the email refers to a whole month (up to the email send date, maybe?) and the dashboard shows the current position, ie the usage since the start of your goodybag, possibly less than a month?
Funny that the giffgaff email tells me that I've chosen just the right goodybag for my usage pattern. If you believe that my usage was over 21GB (as per email) that would be true, but if you believe the dashboard, and I've only used just under 19GB, with 2 days to go till my next goodybag, this advice is obviously poor.