60 seconds and one minute aren't really the same?

This is a bit of a random question, but the answer ha sbeen bugging me for a long time. Ok so...

On a microwave, if you push 60, then it will cook for 60 seconds or 1minute (following me?)
AND also if you press 1:00 it will cook for 1 minute (right? right!)
okay well my question is...
if you add 1:00 and 1:00 is = 2:00 right? so you can cook food for total of two minutes.
BUT if you add 60seconds + 60 seconds it will only cook for one minute and twenty seconds. Even though 60 seconds is 1 minute?
(I know, confusing question, right?)

I guess put into simpliest terms, How come 1 miniute plus one minute = 2 minutes but 60 seconds + 60 seconds = 120 seconds?