Since this is not related to my book directly you're far better off posting this in a genera ASP.NET category here: http://p2p.wrox.com/asp-net-4-5-709/
That way you'll attract more viewers.
That said: it sounds like a CSS issue. If the data i the Grid changes, then the browser can divide the available width across all columns in a different way and / or resize the entire grid.
Try setting an explicit width on the grid ad its columns.
If that doesn't help, please supply a lot more information and code.