This diagram will show you the "life cycle" all gTLDs, especially .coms, follow:
Each registrar has its own "grace period" within the 1st part of that cycle. Once the registrar "lets go" of that expired domain, it follows the latter parts of it.
Notice in the "middle" part that there's a portion where the registrar might try to auction the expired name before pulling the plug. So depending on the registrar of that domain name before, they either auctioned it successfully or let it go and was eventually re-registered by someone else.