HTML5 on iPhone Safari - data stored by localStorage does not always persist. Why?

<script type="text/javascript">
// .001 is the current js version
// this script assumes it is in the root web directory.
var js_file = "site_lib.001.js";
// vars to store where the file is loaded from.
var _mob_load_js = false;
var _mob_ajax_load_js = false;
{
    // if we have localStorage and the files exists there get it.
    if(window.localStorage && window.localStorage[js_file]) {
            // eval the js file.
            try{
                window.eval(window.localStorage[js_file]);

                // successfully eval'ed the code, so 
                // we don't need to download it later.
            _mob_load_js = true;
            } catch (e) { _mob_load_js = false; }
    }
    else if (window.localStorage) {
        // We have localStorage, but no cached file, so we 
        // load the file via ajax, eval it and then store 
        // the file in localStorage

        // To remove previous versions, I remove all of our localStorage,
        // This is extreme if we store other vars there.
        window.localStorage.clear();
        // standard ajax request.
        var xhr = new XMLHttpRequest();
        xhr.onreadystatechange = function() {
        if (xhr.readyState == 4) {
            // eval the js
            try {
                window.eval(xhr.responseText);
                // successfully eval'ed the code, so 
                // we don't need to download it later.
            _mob_ajax_load_js = true;
            } catch (e) { _mob_ajax_load_js = false; }

        try {
                // store the js.
            window.localStorage[js_file] = xhr.responseText;
        } catch (e) {}
        }
        else {
        return;   
        }
    };
    xhr.open("GET",js_file,true);
    xhr.send();
    }
};
</script>

<div id="sr_external_script"></div>
<script type="text/javascript">   
// We haven't loaded the js yet, so we create a script 
// tag and get the script the old fashioned way
if (!_mob_load_js && !_mob_ajax_load_js) {
    var script=document.createElement("script");
    script.type="text/javascript";
    script.src=js_file;
    document.getElementById("sr_external_script").appendChild(script);
    // add a note to the footer
    document.write('<div>loaded from server and not stored</div>');
}
else if (!_mob_load_js) {
    // add a note to the footer
    document.write('<div>loaded via ajax and stored in localStorage</div>');
}
else {
    // add a note to the footer
    document.write('<div>loaded from localStorage</div>');
}
</script>