Summary
The ListPrice from a cart interacts with the exhibited price to show an old versus the current price, in a matter to highlight discounts.
The exhibited price is the SellingPrice, which in addition to promotions, is also automatically modified by a "unit multiplier" if it exists. Like a grocery item, sold in packages of 0.3kg (being the unitMultiplier), where the regular price (eg: $4) is always related to each kg. For this example, the SellingPrice will be "originalPrice*unitMultiplier", totalizing $1.2.
The ListPrice is automatically shown when it's different from (and greater than) the exhibited price. Considering the modified SellingPrice by an UnitMultiplier, the frontend will be showing ListPrice=$4.00 and Price=$1.20, but without being a real discount (but already including one if it exists).
Simulation
Creating a cart with items that match the previous example is enough to recreate the scenario.
Workaround
We recommend hiding the ListPrice on the frontend for items that have a UnitMultiplier.