The headers displayed to you in an email (From,To,Subject, etc) are present only for use by humans. With legitimate email these fields normally reflect reality. With spam that is rarely the case. The mail servers deliver mail using the Simple Mail Transport Protocol (SMTP) and that protocol specifies where the mail is to be delivered - the "To" header is superfluous. Spammers can put anything they want into the "To" field, which is how you receive messages that apparently were not sent to you.

Another common method is to use the Bcc header, or Blind Carbon Copy, to send the messages. Similar to a normal Cc header, the Blind version strips the recipients email addresses from the address headers, allowing mailing lists to send messages to large lists of addresses without cluttering up the email headers with several hundred separate addresses.