If a lead keeps getting assigned to a user other than the user that created the lead, this can troubleshooted by checking the company settings.
Go to Your Roles > Sales Company > Company Settings.
Find the "Set default lead owner?" question to see if there is a rep listed. If there is one listed, that is who all leads will automatically be assigned to after a lead is created.
This can be removed by removing the listed name and replacing it with "Select Option"
This will then allow created leads to remain under the users that created them.