Why only even numbers in the releases

From Norganna's AddOns

All Major releases of any of the Norganna Addon's (Auctioneer, BottomScanner, Enchantrix or Gatherer) have an even number at the end of the release-number.

What is a major release

A major release can mean 2 things:

1. A release that contains that many differences that its first digit changes (e.g. the Auctioneer 4.0 release, which is fully compatible with WoW2.0 & TBC)
2. A major patch release which is fully tested (a X.Y (where Y is even) release)

What are the odd numbered releases

The odd numbered releases are so-called developer releases. Normally you should not use a developer release. The exception is when you want to help the developers test their work. For that you should use odd-number.number release.

When the odd-number releases have been labeled alpha, beta or gamma any user can use them, but still should not be surprised that errors appear. Those releases can also be called minor releases

Developer / Unstable releases

Normally no normal user will use these releases, since they are still under development (thus not complete).
In some exceptional cases you might be asked by a developer (who is trying to fix a problem you've submitted) to verify if a certain developer release will fix the problem. He/she will probably give you the link to the release you should use to verify.