Whenever I open my GMail account and go to see my activity details it show sometimes different IP address with displaying different Cities.
On the other hand it also shows- This account does not seem to be open in any other location, so there is no chance that anyone else is using my account because i always open my account from my home pc only and i also change my password frequently.
Check you IP a few times with whatsmyip or similar site before you access gmail. If you are using a phone to access it your IP will change all the time almost no broadband ISP give you a real IP address and the one you map to changes. It would you think they would always go though the closest gateway but there is no way to really see the network that is being used by the cell providers.