I am running Chrome with a specific --user-data-dir on removable storage that allows me to maintain a single browser session on different machines throughout the day (history, bookmarks, extensions, etc.)

However I notice that login-cookies for most major websites do not persist across machines. Using a SQLite browser and digging through Chrome's database I can see that Chrome has different cookies for each machine I use. I login on Machine A, then go to Machine B and have to log back in on Machine B. When I return to Machine A, my cookie is still valid and I don't have to re-enter credentials.

My question is: What is the extra factor that forces either the browser or the server to require a new cookie? Is it the IP address of the machine? MAC address or something like that?