All of the answers above are plausible/probable and here is what i know happens to me on my tracking...its a simple version I copies from a site because Im too lazy to type it...
"....If you’re using different devices to access Gmail, you may also see more than one device listed. For me, at any point in time, my Gmail account could be open on my desktop PC, my laptop, a Mac, my Android-based phone, or my Amazon Kindle. Depending on my usage pattern and how long Google keeps track, it may look like Gmail is open from five different locations. Coincidentally, all have the same IP address here at my home."
from :
http://askleo.com/should-i-be-worrie...ge-from-gmail/
( I did not twant to create a link - though it would be funny -lol)