@Dave , I can explain the behavior of your R&S scope. I know the answer as to why it doesn't do averaging when going beyond the native resolution of the ADC because I've been spending way too much time looking at these higher resolution scopes. I say "looking" because I can't afford them.
![Sad :(](https://www.eevblog.com/forum/Smileys/default/xsad.gif.pagespeed.ic.L3FGyzQrjB.png)
But anyway, the reason is because they're doing averaging/decimation to get you to that 18bits of resolution. Any further averaging would have a multiplicative effect on the amount of samples that needed to be captured. Likewise, you drop down from 18bits to 16bits as frequency increases because of the same.
I'd like to see the teardown.
I can't post on YT because I don't have a YT account. I deleted it years ago after they started censoring everyone and his cousin's YT channel (even the non-political channels). Though before that, I noted that I had already been "shadow banned" to some extent. This was really weird because I never posted about politics, or any other hot button topic.
Thanks