So I just had this problem myself, and it's strange because it seems to work fine on some tableviews. I'm assuming that the grey color you are using is the background color of your tableview. If not, this solution may not work.
It seems like cells in iOS 7 don't always pick up the tableview background color when a separator inset is present, so you need to manually set the cell background to clearColor. Interestingly, setting this value in storyboards didn't work for me, I had to do it in code. Add this:
cell.backgroundColor = [UIColor clearColor];
when you configure the cell in this delegate function:
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath