It is just another trap for foreigners!
When we are talking about live legs, hands, ears, etc. , thus those that people and animals have it is -AMA in plural instrumental.
When we are talking about table legs, clock hands, pot ears (yes, pot handles in czech are called ears), it is -AMI in plural instrumental.
Just to add some more history to the dual. Old Czech used to have: Singular Dual (two of anything) Plural (for 3-4) Genitive plural (for 5+)
Someone decided that this decided that this was too messy and simplified it by removing dual, and hence today's Czech only have two plural forms. For almost all nouns, the dual-form was removed. But because people were used to speak about certain body parts in two's, for noha, ruka etc. the plural form was instead removed and replaced by the dual form.
And I think the dual form also lives on in somewhat archaic 200: 2 stě.
Hope I got this all right.