Originally posted here by MsMittens
It's not his IP that's being assigned but rather a series of internal caching machines between him and the rest of the internet. A DMZ zone if you will.
This is an old ISP trick, One of my former employers use to do it for our ISDN customers, basically if they all used their bandwidth at once we would be screwed (only had one T1 out and 200 ISDN's) so we would run a bunch of caching servers to save our outgoing bandwidth, this let us oversell our connection. Sure its a little underhanded but its the only way to keep your prices competitive.
Penguin I am pretty sure that this is what is happening in your case as the 10 addresses are non-routable and would not make it past your ISP.