Are 15 seconds 15 seconds?

I have over the last few days found out that 15 seconds not always are 15 seconds, but 25 or just 5...

We've got a pretty standard micro wave in the kitchen. We use it to heat the baby milk frequently these days. It usually takes 45 seconds to heat three bottles to a nice temperature. But sometimes this isn't enough and 10-15 seconds needs to be added. On the other hand 45 seconds may cause the milk to be really hot. I cannot understand why this happens.

45 divided by 3 should be 15.

But interesting here is that when heating one bottle rarely takes 15 seconds. Quite often it takes considerable longer, and just as often much shorter. So heating one bottle is like playing the roulette!

Can some engineer explain this to me?!?