Supposedly refusing orders has no effect. JUNKING an existing order is what causes the reduction.
The 10 to 15 is specifically by fish type. If you turn in a 10 kokanee salmon, supposedly the next salmon order will be 15.
The chance for 2-fish-type orders is supposed to be based on the percentage of fish types you've completed 1-fish orders for.
Similarly, 3-fish-type orders are based on the numbers of fish types you've completed 2-fish orders for, and so on, and requires passing the previous check to go from 1 to 2.
When you get access to a new category of fish, your order quality takes a hit, as suddenly you haven't done all the fish types anymore.
There's a long-time bug that some people get reset back to zeros (and to single fish 10-count orders) out of nowhere. No one knows what causes it.
If you never improve, you might want to contact Mesanna and have someone actually look into your account.