Good question!
'Time' in the login export is when a user actually logs in. It can be the first time a person logs in when completing their registration. Or if they have an account already, its when their session has expired, they load the community and have to login.
'Last visit' in the user export is when you have an active session already and you load the community. For example, you have clicked on remember me when logging in, so a cookie is placed in your browser, you have closed the community tab in your browser, you open a new community tab and load the community. That is counted as a visit as you never had to login.
