some companies/organisations in USA still hold IP ranges that are routable. A large tire company located in USA, for example, has 2 class B networks. Although its uncommon nowadays, you location may be one of those.
However, diferent pc may report diferent "internet IP" also due to:
a) there is a IP pool and proxy is balancing between them
b) there is More than one proxy at your network, each of them with on "internet ip" address.