This is one strange bug. I have a simple RadGrid with a couple of columns, I use a GridTemplateColumn with a checkbox control for deletion, heres the markup.
Anyway, I have a button that does a full postback that checks for .Checked checkboxes and executes the delete operation, like this.
All works as expected, only the checked rows are deleted and since I reload the grid again, no items are checked once the page has reloaded, it all looks ok until you refresh the page again. If your grid had say 10 items and you ticked the first item and deleted it, as expected the grid now would have 9 items. If you refresh the page, a full postback is done as expected however, the first item is marked as "Checked" when it was not, so the item is then deleted and how you have 8 items despite the fact that you had not explicitly checked any item.