Microwaves are confusing

On a microwave if you push 60 then it will cook for 60 seconds or one minute and if you press 1:00 it will cook for one minute or 60 seconds right? If you add 1:00 + 1:00 it is = 2:00 so you will cook for a total of two minutes right? BUT if you add 60 + 60 seconds it will only cook for one minute and twenty seconds. even thought 60 seconds is one minute? how come 1 minute + 1 minute = 2 minutes but 60 seconds + 60 seconds = only one minute and twenty seconds?