Comparison operators (equal '=', not equal '<>') doesn't work as expected with references to empty cells. For example, expressions of type '=IF(A1="", TRUE, FALSE)' are evaluated to false when A1 cell is empty. 

In MS Excel, such expression is evaluated to TRUE if the cell is empty - i.e. empty cell value is considered equal to an empty string "".

Workaround: Use ISBLANK function if possible. Its results are consistent with MS Excel.