Didn't want to leave your post unanswered. My doc explained to me that the "day 21" test should be about half way through your luteal phase. So it would only be 7 dpo if you have a 14 day luteal phase ("day 21" is just shorthand for this as it's based on that mythical woman with the 28 day cycle who ovulates on day 14!).
I was also told to try and get it as close as possible mid-luteal phase as too early and your prog levels may not be high enough; too late and your levels may have started to drop. Both being too early or being too late would give misleading results.