Ok, so I think I have an answer. After scraping around help documentation it appears that Shodan is focused on certain services. It does not appear to do a full port scan of every IP regularly, only specific common ones like telnet, ftp, http, and etc.
It does capture more than just those ports through other mechanisms, but it is not a full internet every IP address with every port database. I found this information on a Defcon power point, but it is also alluded to in the wiki which is more trustworthy:
"Shodan collects data mostly on web servers (HTTP/HTTPS - port 80, 8080, 443, 8443), as well as FTP (port 21), SSH (port 22), Telnet (port 23), SNMP (port 161), IMAP (port 993), SIP (port 5060), and Real Time Streaming Protocol (RTSP, port 554). The latter can be used to access webcams and their video stream."