what's the point of 30% new material?

I decide to convert my KDD08 paper into a journal version. Unfortunately, it is said that I have to add at least 30% new materials.

The sad thing is that I have wrote too much for the conference paper, since I want to make the paper more solid and comprehensive.

But it turns out I have nothing to add for the journal paper(Of course, you can always add something new but the cost is way too high, almost another journal paper).

So that's what I learned from this class:
Don't write too much in one conference paper.

It's really funny and ridiculous.