What happened to 30 cent?

Heyy there,

Today I got a sale with referral. The item list price is $59. Normally referral cut is $18.30 on a $59 item. But today I got $18. So there is a missing 30 Cents.

My last referral sales was 10 days ago and it was again $18.30. It was always $18.30. Have there been some changes on referral rates? Or is there a bug on the statement?