I have a jqGrid on a page wrapped inside an ASP.NET web part. Here is it's definition:
$("#referent_grid").jqGrid({
    url: '<%= SPContext.Current.Site.Url %>' + wsBaseUrl + 'ReferentService.asmx/ListReferents',
    colNames: ['Full Name', 'Phone Number', 'Email', 'Department'],
    colModel: [
        { name: 'FullName', index: 'FullName', width: 240, align: 'left', search: true, stype: 'text', searchoptions: { sopt: ['eq', 'bw', 'bn', 'ew', 'en', 'cn', 'nc']} },
        { name: 'PhoneNumber', index: 'PhoneNumber', width: 120, align: 'left', search: true, stype: 'text', searchoptions: { sopt: ['eq', 'bw', 'bn', 'ew', 'en', 'cn', 'nc']} },
        { name: 'Email', index: 'Email', width: 180, align: 'left', search: true, stype: 'text', searchoptions: { sopt: ['eq', 'bw', 'bn', 'ew', 'en', 'cn', 'nc']} },
        { name: 'Department', index: 'Department', width: 180, align: 'left', search: true, stype: 'text', searchoptions: { sopt: ['eq', 'bw', 'bn', 'ew', 'en', 'cn', 'nc']} },
    ],
    jsonReader: {
        id: "ReferentID"
    },
    pager: $('#referent_grid_pager'),
    sortname: 'FullName',
    sortorder: "asc",
    height: '300',
    shrinkToFit: false,
    caption: 'Referent List'
});
$("#referent_grid").jqGrid('navGrid', '#referent_grid_pager',
    { add: true, addtitle: 'Add Referent', edit: true, edittitle: 'Edit Referent',
      del: true, deltitle: 'Delete Referent', refresh: true, refreshtitle: 'Refresh data',
      search: true, searchtitle: 'Advanced search filters',
      addfunc: addReferent, editfunc: editReferent
    },
    {}, // default settings for edit
    {}, // default settings for add
    { // define settings for Delete 
        mtype: "post", reloadAfterSubmit: true,
        url: '<%= SPContext.Current.Site.Url %>' + wsBaseUrl + 'ReferentService.asmx/DeleteReferent',
        resize: false,
        serializeDelData: function (postdata) {
            return JSON.stringify({ referentID: postdata.id });
        },
        afterSubmit: function (data, postdata) {
            var result = $.parseJSON(data.responseText);
            return [true, ''];
        }
    },
    { closeOnEscape: true, multipleSearch: true, closeAfterSearch: true }, // search options
    {}
);
As you can see I have Advanced Search enabled. The problem I am facing to is that the first time the page is called the jqGrid calls the ListReferents method without passing the filters parameter as you can see in the following screenshot from Fiddler

When I click on the refresh button of the jqGrid it calls the ListReferents method passing the filters parameter as you can see in the following screenshot from Fiddler 

To arrange this I have defined two methods inside my web service but the first method does never get called while the second is.
[WebMethod]
public JQGridData ListReferents(string _search, string nd, string rows, string page, string sidx, string sord) {
    return ListReferents(_search, nd, rows, page, sidx, sord, string.Empty);
}
[WebMethod]
public JQGridData ListReferents(string _search, string nd, string rows, string page, string sidx, string sord, string filters) {
    // method code here
}
Where am I doing wrong?