TGridPanelLayout Row and Column size setting

Does anybody have any clue about setting the row and column size values in TGridPanelLayout precisely? If you place a TGridPanelLayout on a new form it will have 2 rows and 2 columns set at 50% all round. If you then select Row 0 and change it’s % value to 20%, the value changes to 26.571%???
Why? There must be some weird rules about doing this simple process but for now it makes no sense.

Thanks, Gerard