Explanation of cookie behavior

Gurus, I'm running a login script where the user enters login/password and if it matches what I have in my db and their account is active, I set a login cookie (login_ck) and an authentication cookie (authenticate_ck). If the login and authentication cookies are set when the user goes back to the login page I prompt with "welcome back.". Now, I refresh the login page a few times, sometimes it gives the welcome back prompt. Then after anywhere from 2-5 refreshes, it deletes the login cookie but the authenticate cookie persists. Any ideas why this is happening? I'm setting my cookies like this: setcookie ("authenticate_ck", "$daysRemaining"); //set cookie for active account, for 30days setcookie ("login_ck", "$lo", time()+ "60*60*24*30", "", "", "0"); //set cookie for login, for 30days Thanks, Ryan