It's a cache. So once you've shown the log on your machine, all log entries that you show are then stored in the cache so the next time you show the log, those entries won't have to be downloaded from the repository server anymore.
And that's why you won't see any changes/edits to those already cached entries.
Since log messages should be edited very, very rarely if ever, you only have to do this when an edit happens. (Or hit Ctrl+F5 in the log dialog - that will re-download all cached entries as well).