That achievement only counts if you boost it up a level. For example, if you boost an item from Power Level 1 to 2, that would go up from 42 to 43.
Yes, it's worded quite poorly, but that's how it works.
Tip: Use high power junk items to fuse into other high power junk items to raise it fast, but make sure those items have the same power requirement at level 1. (300 for mythical items and level 30 items)