When a process attempts to connect to a particular IP address, Little Snitch uses a heuristics (based on previous DNS lookups, Deep Packet Inspection and more) to determine the corresponding hostname for that IP address to figure out which of your filter rules shall be applied.
In some rare cases this approach may be unable to find a unique hostname match for a given address. The connection alert will then only show the IP address for that connection, often together with a list of possible hostnames which all resolve to that same address.