When a domains expires, and isn't renewed by the owner, it continues to be held by the original reseller. (A reseller is a company that purchases domains with the intention of selling them rather than consuming or using them. Most domains are originally purchased by resellers, rather than individuals.) The reseller can keep this domain until he or she is ready to release it to the general pool.
To view an accurate status of a domain, visit DNSstuff.com. Once there, scroll down to the "WHOIS Lookup" section and enter a domain. You'll be able to see details about the registrar, reseller, status, when the domain was purchased, and more.