Some file revisions only visible under 'Logical' mode?

I'm not sure if this is a bug, or just behaviour that isn't explained well or understood by me.

I attempted to browse to the following URL: https://svn.atlassian.com/privateeye/browse/private/atlassian/jira/trunk/project.xml?r1=57229&r2=52750
This is with the profile option 'File history view mode' set to Physical
Note the file revisions:
52750 and 57229

The revisions that actually show up in the diff are different:
52520 and 57168

When I change the profile option to Logical, the diff shows the correct versions when visiting the previous URL.

Can I ask why this is the case? It's slightly confusing.