Membership expiry question (Fearless Blue)

What should happen when you extend your membership (by paying USD to gain membership days and GC)?

I was expecting that the membership days would be added to those still remaining, but when I look at the transaction log, it looks more like the extra membership days are put "on top" of the previous ones, so the membership period is X days from when you paid last, not X+r (where r is the number of days remaining from your previous purchase).

Anyone know the fine details of this?

just bumping this as Tom is active on the forum at the moment.