Interesting - I hadn't heard this before. I'm guessing that if it's true it's due to the fact that at a higher AUM then more collective pitch will need to be pulled in order to maintain the given NR. More pitch, more lift, lower ROD.
It's a funny relationship, the fact that at 90% rpm in auto the ROD is usually lower than at 100%. However given that in the lift equation the V (mostly rpm) is 'squared' one would think that the lift would be noticeably reduced at lower RPM and therefore the ROD higher? However in increasing pitch to reduce the RPM the increase in coefficient of lift seems to outweigh the reduction in Vsquared... the result being a reduction in ROD. Perhaps someone can clarify further as it seems a little counterintuitive.