Probable cause:

  1. Ajax/ Drawer Cart
  2. Any other sales channel other than Shopify product page like Facebook or any remarketing/retargeting tools/apps that bypass Shopify product page like Permalinks to pre-load the cart
  3. Third-party product option apps that use ajax
  4. Upsell apps
  5. Lazy loading
  6. A quick view in the collection page or similar product list or recently visited product list
  7. Additional Popup Add to cart button