As with a lot of color management functions, consistency is the key.
The Advanced mode has the user going through the process of turning the brightness up and down, while the Easy mode by passes this and merely calibrates the display at whatever brightness it happens to be at. One of the reasons why some high end displays have a DDC feature is that it takes away the chance of any user-introduced variability.
Looking over your post again, I see that you’re dealing with a previous-generation iMac and trying to calibrate to 120 luminance. Those iMacs will almost never be able to get down to 120 using only the Mac brightness adjustment. Most iMacs I’ve seen end up with a luminance of around 180 - 200 or so, even with the brightness all the way down. I can see where having the brightness this far down would cause some variability also.
We had an article in one of our newsletters that mentions this issue:
colorwiki.com/wiki/My_Printer_Is_Too_Dark