Where on that site are the delivery requirements?
Odd that they would attenuate a file by 3dB if it is only 0.1dB over the limit! Why would they not simply attenuate the file by 0.1dB? You mention above that you have to deliver at a peak of -12 dBfs and RMS -17.5. How do you set up the RME meters to measure a ‘strict’ -17.5 RMS value?
As hinted by Fredo, EBU-R128 is the future of commercial delivery, not RMS meters. But if you would like to encourage Steinberg to change the RMS value display you could perhaps try making a feature request. AFAIK nobody here is suggesting that Steinberg won’t bother. They may well be interested if you present a feature request in the right way.
They are all using IEC 61606:1997. The differences are in the use of the statistics and the ballistics, not the standard (as mentioned above). By default the RME RMS value display is an instantaneous real-time RMS value and the Nuendo value display is an RMS Max value.
P.S. You mentioned above:
That’s not true. The Nuendo meter does reach the quoted RMS max at some point in the audio you have played. The same RMS max figure is confirmed if you use Statistics.