Is your web server being NAT'ed? firewalled? proxied? There are a bunch of network devices that can do funky things with incoming traffic and headers.
Also, what does the code that is recording the IP addresses look like? Where are you getting the addresses from? If whatever you are doing is grabbing the wrong part of the header you might be getting the last routers IP addr as the source, in which case it will be the same almost every time (only a couple unlikely scenario's I can think of where it would be more than 1)
There are alot of things that could be happening here, and without a little more topo info, as well as info on where this log entry is coming from, it's quite hard to say what is going on.
"Nothing is sure but death and taxes" I say combine the two and its death to all taxes!
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