为什么有些chrome下的cookie值使用js移除不了？

移除cookie的方法使用的下面链接中的方法
http://stackoverflow.com/questions/21443...

function createCookie(name,value,days) {
    if (days) {
        var date = new Date();
        date.setTime(date.getTime()+(days*24*60*60*1000));
        var expires = "; expires="+date.toGMTString();
    }
    else var expires = "";
    document.cookie = name+"="+value+expires+"; path=/";
}

function readCookie(name) {
    var nameEQ = name + "=";
    var ca = document.cookie.split(';');
    for(var i=0;i < ca.length;i++) {
        var c = ca[i];
        while (c.charAt(0)==' ') c = c.substring(1,c.length);
        if (c.indexOf(nameEQ) == 0) return c.substring(nameEQ.length,c.length);
    }
    return null;
}

function eraseCookie(name) {
    createCookie(name,"",-1);
}

整个cookie串如下，其中PD-ID移除不了，PD-H-SESSION-ID可以被移除。

PD-; PD_STATEFUL_49f8a770-2968-11e5-a347-00215e549df4=%2Fvlcmain; PD_STATEFUL_76302938-ad09-11e5-8871-00215e549df4=%2Fzjzf; PD_STATEFUL_f8a4a5fc-45a2-11e5-b6d3-00215e549df4=%2Fvlctzht; AMWEBJCT!%2Fvlczlgk!JSESSIONID=0000j_HQefhvDPTudEdMzbW22Eo:19qb1pju2; PD_STATEFUL_3ddcd51e-4d22-11e5-8189-00215e549df4=%2Fvlczlgk; IV_JCT=%2Fvlcmain; AMWEBJCT!%2Fvlcmain!JSESSIONID=237389B056659DE68090D69B5B902F4C; JSESSIONID=0000Vw5EwTQB-4jEfXlQWcNXXsC:19qb1ro44; PD_STATEFUL_947cbe14-f017-11e5-a6b9-00215e549df4=%2Fvlczb; PD-H-SESSION-ID=4_8nov9TVEGEfTH-kZUtomzC4ZQXZrfZ5MJbDGA82RYz0jMvPI; PD-ID=slbdeMMOUjAhiNOMMQJbZFAnR9fANeii386VFfeWRDk377BZcH1CYOtNUmm1bRQ7SXpDJhUQSnt0keJwgyGSoTPLSkGuYLt5lgp8D5B4eZeSGoHxWWabas1zyAVKkkLxLFlnwqYoM2CjenGpExy1bF3SAxiK429u+3bsl/8f9NAdQSKQiP1OH8qAxi+S4BCrokCu//kflpN8Xa7/qw8wMyWP7NzOLmFEmjoggETkNx/MDJcYNj8dYyHMHscCb7CzUVuWC870OuyoASBmgcjEBVKht49lYggL