Develop displays histograms for identical images with small but noticeable differences, and these differences depend on the order in which you edit the files. By itself, this probably doesn't have any impact on users, but it may indicate a worrisome underlying bug in how Develop caches computations.
See this screen recording, and follow these steps precisely:
1. Download this sample raw into a folder and make a copy of the raw in the same folder. Rename the two raws A and B.
2. In LR, do Preferences > Performance > Camera Raw Cache Settings > Purge Cache.
3. Restart LR.
4. Import A and B.
5. In Library, select A and make a virtual copy.
6. In Library, select A and Cmd-click B (order is important).
7. Go to Develop, and use the cursor keys to switch between A and B. The histograms appear identical.
8. Go to Library.
9. Without changing the selection, go to Develop, and use the cursor keys to switch between A and B. The histograms now have small but noticeable differences.
10. In Library, select A and Cmd-click the virtual copy (order is important).
11. Go to Develop, and use the cursor keys to switch between A and the virtual copy. The histograms have small but noticeable differences.
Tested on LR 8.3.1 / Mac OS 10.14.5.