As jvamos says the Nuendo meters have a slower RMS response time, and as Fredo says the Nuendo meters are showing the RMS Max and not an average historical RMS. AFAIK the Digicheck meters are not showing an RMS Max. So you’re not going to get the same result from both meters.
It might be better if the Nuendo meters had the option to show an average historical RMS value rather than the RMS Max, but if it’s of any use to you you can get the average figure in Nuendo by using the Statistics function.
You’d get a more accurate result if you used the loudness meter instead. You could try to encourage your TV station to give you their delivery requirements for a loudness meter rather than an RMS meter.