Why Billing adjustments - additional Seat updates $15.48?

I started a month to month collab plan $5 just last month, and looking to continue to month 2. My next billing reflected a “Full” plan which I did not sign up for. When referring it back to my original collab plan of $5 for next month, it adds this “Billing adjustments - additional Seat updates $15.48?”. How is that possible?  Do you mean to tell me that I’m getting charged an extra $15.48 for continuing a month 2 on a month plan (which I’ve not started yet) or is this just a bad UI issue? Please assist.