Hey Trevor, I absolutely agree that the same term for two different meanings is very confusing, I suggested to Tuan a few years ago that the term 'Grace Period' on the Plan Detail page should be re-named, and I suggested "Bonus Period" as a better option as I think this better describes the period where an additional bonus membership period is given for new subscribers.
The generally accepted meaning of a 'grace period' is a period where you allow something to continue after a deadline has passed, for example a bill has not yet been paid. This is the correct use of that term on the General Config page.
I recall Tuan agreed on this change but he hasn't implemented it as yet.