Your approval percentage is based on your last 500 premium decisions. It does not include the free (standard) submission, or those submissions that have "expired" because more than 48 hours have passed.
Let's super simplify this and say it's based on 10 responses instead.
Here are those decisions, where "n" means you declined and "y" means you approved (yes/no).
(oldest) n n n n n y n n n n (newest)
9 declines (n) and 1 approval (y) means you have a 10% approval rate.
Now let's say you decline two in a row... your history changes to:
(oldest) n n n y n n n n n n (newest)
You've added two "n" / declines on the newest side, and 2 of them have been removed from the oldest side.
But your approval is *still* 10% because your 2 new declines have simply replaced 2 old declines.
Hope that helps explain why simply declining 5 in a row will not always impact your approval percentage — especially if you've been on here for a long time!