I get that group 14 elements show +4 and +2 oxidation states so they can form both ionic and covalent compounds and so they are amphoteric but monoxide means that these elements have to show +2 oxidation state in these oxides and dioxide means that the oxidation state of these elements will be +4.
Monoxides of ge,sn and pb should be ionic because of fajan's rule and dioxides should be covalent then why are they amphoteric? I know that it is because they react with both alkali and acids but what is the reason for such behaviour?