I think the reason might also be that gmail supports POP3. if you check your gmail messages from home they won't show up on your office machine. gmail doesn't have an option (that I know of) to "leave messages on the server". All gmail messages are downloaded to the local machine checking the email and then deleted off of gmail's servers, thus, not allowing you to check messages on multiple accounts. IMAP could act as a workaround because it supports leaving messages on the server. With IMAP, you can download emails on multiple accounts because all emails are stored on the server until deleted by the user.
Hope this was helpful!