Lightmapping.buildProgress separated into multiple parts?

So, the progress value that goes from 0 to 1 seems like have a multiple parts so it goes from 0 to certain number and then goes back to 0 and then up again.

I can understand that this behavior, if the progress is divided into parts. But there is no way to tell what parts the progress indicates. This makes hard to make use of the progress value meaningfully.

We either need to have some call back for each parts, or have some kind of enum indicating what parts we are in so that the progress value can be meaningful.

Anyone has any idea if I am getting it wrong?