IndexWriter.getReader.getVersion behavior

[Peter Keegan]

Using Lucene 2.9.1, I have the following pseudocode which gets repeated at
regular intervals:

1. FSDirectory dir = FSDirectory.open(java.io.File);
2. dir.setLockFactory(new SingleInstanceLockFactory());
3. IndexWriter writer = new IndexWriter(dir, Analyzer, false, maxFieldLen)
4. writer.getReader().getVersion();
5. writer.prepareCommit();
6. writer.getReader().getVersion();
7. writer.commit();
8. writer.close();

I'm using the version number to keep external data in synch with the index.
Usually, the version number from (6) is 1 greater than from (4) and the
version from (4) equals the version from the previous (6). At least once a
day, however, the version from (4) is 1 greater than from the previous (6).
What would explain this sporadic behavior of version numbers?

Thanks,
Peter