I have just tested using two google accounts. If I sign into Google with an account then sign out, the account is added to the Google account sign in list. I have done this with both accounts and being signed out I am now presented with a choice: account 1, account 2 or use another account.
It seems you must have been signed in with a google account to the webviewer previously for the accounts to be offered. ( I am guessing this is the case with your native browser, which is why you see multiple accounts)
You get the same behaviour if you just go to a google page to sign in to google.