The way I understand the patch note info:

You gain skill points for desynthing, that much is common knowledge.

Whilst working on a particular skill, you may also experience other skills going down. This is the random loss you are seeing.

When the combined total is at 300, then you will experience other skills going down.

The difference lies in the may and the will parts. One implies a chance of seeing the downlevel, the other implies a guarantee of seeing the downleve.