1. If one enters edit mode by pressing F2, clicks to enter edit mode or comes from another cell in edit mode - the value in the cell that enters edit mode is selected and any user input removes the old value.
2. If one enters edit mode by directly hitting a numeric key the value in the cell is set to that numeric value, a decimal separator and a number of trailing zeros equal to the editor decimal places.
Example:
Let a cell value be 1.23
If one wants to input 4.56 (s)he will have to press 4 then select the trailing zeros and press 5 and 6.