jquery jqgrid "page index" logical error

See more:

C#

jQuery

JQGrid

ASP.NET4

Hello Experts,

(Web application information- I am using ASP.NET MVC, .Net framework 4 and C#)

I am using jqgrid on a webpage(view) to display contents from model.

$("#list").jqGrid({
            url: '<%= @Url.Action("ViewData", "Data") %>',
            datatype: 'json',
            mtype: 'GET',
            ...
            ...
            ...
            toppager: true,
            pager: jQuery('#pager'),
            rowNum: 10,
            rowList: [5, 10, 20, 50],
            sortname: 'ID',
            sortorder: "desc",
            multiselect: true,
            viewrecords: true,
            ...
            ...
            ...

and my controller is this

[SessionExpireFilter]
       public ActionResult ViewData(string sidx, string sord, int page, int rows ,GridSettings grid)
       {
           // code goes here..
           // return JSON object.

       }

I have applied Virtual Scrolling, so whenever the user will scroll on the page, the data will be loaded in jqgrid.

PROBLEM:
When I debugged the application, the "page" parameter in controller must show incremental values at each scroll but it is not behaving normally.
-If I keep the "sortOrder" as "desc" then the "page" values I get in controller are '1' in first pass, '3' in second pass, '5' in third pass and so on.

-If I keep the "sortOrder" as "asc" then the "page" values I get in controller are '1' in first pass, '2' in second pass, '5' in third pass, '6' in fourth pass and so on.

I have tried my level best to find out where the problem lies but I am not able to find the reason behind this. :(

Any help would be highly appreciated.
Thanks in advance.