When you add leading white spaces in table cells in Visual Editor they are shown in the editing mode, but removed when you save your edit. This is confusing. A suggestion is that the leading spaces, when saving, should be changed to . Or that a notification/warning is shown.
The same behaviour take place when you add leading white spaces in the beginning of a sentence/paragraph. In wiki text mode the space is saved (but not shown in reading mode). However, when using Visual Editor the white spaces are completely removed when clicking save (also when you go back to edit in wiki text mode).