Most commercial products run a fairly set course; you get Microsoft Office 1.5, then 1.6, 3.0, 4.0, all the way up to office 2010 (For Windows, at least). It's a fairly orderly progression, with version numbers rising over time – pretty easy to follow.
Open-source projects are a different beast, however. When enough developers don't like the direction a project is going, they sometimes just create a fork and go on developing their own product. Sometimes projects have to fork for legal reasons, and sometimes it's a business decision, too.
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