Q: username/password log me in to different account

This issue happens from time to time (once a week at least). I turn on my Macbook Air (Model: 6,2) which has the latest update of the OS (El Capitan). I select my username and proceed to enter my password, but then it decides to log me into another (password protected) account. Now I know Apple is all about security for their users, but it seems like this may be something they may have not planned out too well. Unfortunately, I don't not have a recording of this happening. but I will try to update this post with one if I can capture it.

The OS version I am running is: 10.11.13