There are at least two reasons why there can be more Referring IPs than Referring Domains.
The same domain can have multiple IP addresses. Usually, this is done to spread the load. More information: http://en.wikipedia.org/wiki/Round-robin_DNS
Let's suppose X is the domain for which there are only three backlinks from these pages:
http://subdomain1.domain.com/index.html is located on the server with IP 188.8.131.52
http://subdomain2.domain.com/index.html is located on the server with IP 184.108.40.206
http://subdomain3.domain.com/index.html is located on the server with IP 220.127.116.11
It turns out that Domain X has only one referring domain (domain.com) but 3 referring IPs (18.104.22.168, 22.214.171.124, 126.96.36.199)