What cart abandonment means
Adding a product to the cart expresses a concrete intent to buy, firmer than a visit to the product page. If the session ends without a transaction, that intent stays on record and unpaid. The gap between the two is what the metric describes.
The analysis goes wrong at almost the same spot every time: confusion with checkout abandonment. These are two different stretches of the same journey. Cart abandonment covers everything that happens once the product is in the cart, including comparing prices in another shop or keeping the cart for another day. Checkout abandonment starts later, once the user has begun the payment process, and only covers the screens for details, shipping and payment.
In practice this means the two figures never match and are not interchangeable. Cart abandonment always comes out higher, because it also picks up people who use the cart as a wish list or as a calculator for the final price with shipping. Checkout abandonment is lower and more expensive: by then the user had already decided to buy and something stopped them. Mixing the two figures assigns the payment provider a problem that sat on the product page, or the other way round.