Why do only some abandoned carts show up in my abandoned cart list?

A cart is only captured once there's an email to send the reminder to. The shopper has to have items in the cart and type a valid email address into the email field on your checkout page. Visitors who leave before entering an email can't be emailed, so they aren't listed.

Also check:

1. Abandoned Carts → Settings: Enable abandoned cart capture & recovery emails is ticked. When it's off, no carts are captured.
2. The Abandoned Cart Recovery plugin is active for your store.

The Overview tab shows the 10 most recently captured carts.