This feature is available in all editions.
All backlog items aren't displayed
Check to see if Closed items have been filtered out.
Check other filters if this doesn't fix the problem.
Items moved to or from another sprint
Done amounts can differ if Effort was logged in one sprint and the item is subsequently moved to another sprint.
Item was deleted after hours were logged to it
Prior to being deleted, an item could have had hours logged to it. That would leave the hours in the Sprint Summary without showing the details in the grid.