Shows a possible way of working around the apparent bug in SP2013 apps, where the SPHostUrl parameter value is incorrect on subsequent navigation to the app default page.

SP2013App_SPHostUrl_BugWorkaround.js

function sharePointReady() {
    // retrieve passed app web/host web URLs..
    if (_spPageContextInfo != undefined) {
        // allow this code to be used in an app part page..
        appweburl = _spPageContextInfo.webAbsoluteUrl;

        if ((document.referrer !== "") && (!document.referrer.toLowerCase().startsWith(appweburl.toLowerCase()) && ($.getUrlVar("SPHostUrl")))) {
            // we came from outside app web and are being passed SPHostUrl, so this should be the correct value - let's 
            // store it in a cookie against this path..
            hostweburl = decodeURIComponent($.getUrlVar("SPHostUrl"));
            var x = appweburl.replace('://', '');
            var cookiePath = x.substr(x.indexOf('/'), x.length - x.indexOf('/'));
            COB.cookieHandler.setCookie(cookieName, hostweburl, null, cookiePath);
        }
        else {
            hostweburl = COB.cookieHandler.getCookie(cookieName);
        }
    }
    else {
        hostweburl = COB.cookieHandler.getCookie(cookieName);
    }
}