The unmatched in IP to AS conversion
To account for the 20% unmatched IP addresses in the conversion, one explanation could be found in [1]. The authors claim that “there are at least about 25-50% more AS connections in the Internet than commonly-used BGP-derived AS maps reveal.”
Those unmatched IP address from CAIDA could most likely be found among ASes in two other datasets: Internet Looking Glass sites and Internet Routing Registry (IRR).
The Internet Routing Registry (IRR) currently provides the only mechanism for validating the contents of a BGP session or mapping an AS number to a list of networks: http://www.irr.net/.