Explain my microwave oven's strange behavior.

It’s not really strange, just something that I don’t understand and am curious about.

It’s a new microwave I purchased recently. I never noticed any previous microwaves behaving like this. I’ve only noticed it when running at low powers, for example when defrosting something (but not necessarily using the defrost button).

Normally, when the counter hits 1, it goes back to displaying the time, shuts off, and beeps several times. However, at low powers, when it hits 1, it beeps twice, then continues running (at least the light, is on, the fan runs, and the plate rotates) and the display jumps to 4 then does a quick countdown (less than a second per number) from 4 to 1 before going through the normal shutdown.

I assume that this is intended behavior and not some software or hardware bug. Why does it do this?