This happens because the login specified has a different sid on the primary and secondary servers. The logins are mapped to database users by sid. When you restore a log backup on the secondary server, you reset the sid to the primary login.
You must create a procedure, in the master database, to script a login. This script will contain the login, including the sid the primary uses. Then run the login creation script on the secondary server. Now each login shares the same sid. This sid is assigned to the same user in each database.
Everything you need is in this link