The 6 days is I believe leeway they add on to take into account time lost when it's in the post to you. If they've post dated it that isn't really applicable, but it must be a standard process.
It seems reasonable.
I just bought a Two Together Railcard, ordered on 4 February for a journey booked for 22 February - two of us will be travelling and we'll save more than the £30 railcard cost by using it.
Ordered online and delivered today, 11 February.
Validity until 10FBY23, so that looks like 6 extra days.
Previously I recall the railcard taking less time to reach me, but it looks like 6 days is sensible, and looks like a standard process for online/post railcards.