There is no +60 button there is a +1:00 min button.

Microwaves only see in mins and seconds. If it’s the first two digits it’s mins and the last two are seconds.

So 00:99 seconds is longer then 01:00.

Microwaves never leave off the leading zeros they can’t. Least no microwave iv ever seen in 20 years can and iv seen microwaves that are from the 70s.

They basically just have always worked this way.