You think, but I know! You're absolutely 100% correct!
If an aerodrome is situated 2.000ft AMSL and you're supposed to correct a step-down altitude of, say,
4.000ft, you'll only have to correct the 2.000ft AAL. Thus, a 10% correction would mean +200ft - not +400ft!
Try looking at your RAD ALT next time you fly into an AD at high PA (assuming the surrounding terrain is flat...) and you'll see for yourself :-)