This is to make sure the price is accurate. When you’re shopping, the product price you see is what’s stored on your Android device. When you add a product to your trolley, and when you check out, the app syncs with ocado.com and updates any prices that may have changed.
The app only syncs prices once they’re in your trolley, to keep the app running smoothly. There are tens of thousands of products in the Ocado catalogue, syncing them all simultaneously could slow the app down.