Not seeing referral fees be charged sometimes FBA Seller

Something weird is happening on my account where occasionally for some of my products I do not get charged the referral fee at all.

Has anyone experienced this or know why it happens? I am not complaining I am just worried they have some algorithmic issue and it accumulates and they charge me later haha.

I tried searching online and on their forums and I have not seen anyone experience this “issue”. My possible is incredibly lightweight, small, and around 9-10 dollars. I have ad campaigns as well for them. Not sure what it could be, appreciate anyone’s input here.