Roger, I think everything is fine.
Using a cursor placed on my plots, I can step through the Return Loss for each point. Here's what I see for the last 10 points for each capture:
Nano 8753
------------------
10.98 10.97 <- 273.27 MHz
10.98 10.97
10.97 10.97
10.96 10.96
10.96 10.96
10.98 10.96
10.98 10.96
10.96 10.95
10.95 10.95
10.94 10.95 <- 300 MHz
Discounting the rounding, these seem to trend pretty close (to me) to the data shown in your plot. And note that the Nano data is noisier than the 8753C data, which matches what you show, too.
- Jeff
