The editor-view IPR for a fast preview. If you like to have something reliable, use the Picture Viewer.
They might come close, but the data is different; it depends, of course, on your monitor and how much you can see.
Render in the editor view. Then use the
Editor> View> Send to Picture Viewer.
Now render directly to the Picture Viewer,
Click to see the Histogram. Can you see how different both are?
Yes, you pointed this out already. But did you see that huge of a difference? In other words, a little more is going on to set the IPR up for speed and the Picture Viewer for quality.
If you think that this, while it is since over two decades so case to be a bug, please check with tech support
Enjoy your weekend