The Buyer IP we report in our Transaction Log and in order-data submissions using our Integration feature is obtained from the buyer's cart when they proceed to checkout. How are you cross-referencing a particular login session on your site to any particular cart order? It may be that you're mismatching a login IP to an unrelated cart order.
Also bear in mind that many users' computers will get assigned a new IP every time they connect or start using an always-on connection after an extended lull, out of a shared pool of IPs their ISP maintains for their customers to use (this is called DHCP). Conceivably, if your users are allowed to stay logged into your site with something like a persistent login-session cookie, if you're logging their IP when they first log in, they may have a different IP by the time they take their cart order to checkout.