It is possible for you to get multiple Visitor IDs for the same user.
The identification method for these two types of visitors is not the same. One is based on a cookie on the visitor's browser while the other depends on information you push into the hit or into the event on your end.
If the same visitor uses your site from their personal computer and later from their work computer, if they used the same account to log in both times they will be recognized as the same user yet count as distinct visitors.