It's actually a cosmetic display bug. It happens when you connect to a server that doesn't have an IRC connection. There is no way for it to determine the actual list of servers available and falls back to the hardcoded list built into the client. The servers hardcoded are all shown as full servers however that's just because the built in list doesn't differentiate between the two types. I tried to fix this once, but the fix was too complicated and we reverted it.