Three prices, and the one that lands on the line
An item carries three amounts that answer different questions. The selling price is what you ask. The cost price is your margin reference. The supplier price is what that particular supplier charges you, with their lead time and minimum quantity.
The document takes the one that concerns it. A purchase line starts from the chosen supplier’s price; a sales line starts from the selling price, replaced by the customer’s price list where they have one. The line says which one served, so an unexpected price explains itself without opening the record.