I've been having the same issue recently when booking flights between LCY and TXL: the total fare shown has been consistently higher than the sum of the two selected fares. The difference is always around €9, so it's not the Plus fare that is being shown, and persists even when I start over and select the flights again.
It's annoying but, as with most things on ba.com these days, I just put it down to poor IT and accept that BA will not be in a hurry to deal with it.