Maybe I don't understand something...

Yes, that's right. The team behind the mod got a license from VALVe to use the Source engine and make a full commercial game. From what I read, is more or less the same as the mod, although the Steam version is updated.

Yup the steam version is redone and updated

From Desura:

thechineseroom have announced that the remake of the critically acclaimed, award-winning mod Dear Esther will be getting a full commercial release in summer 2011.

The groundbreaking game, which replaces traditional FPS gameplay with the exploration of a desolate island, a deep and emotional story and a stunning soundtrack, will be launched on Steam later this year. Over the last twelve months, the award-winning mod (Indiecade 2009, Mod of the Year 2010) has been re-developed by Robert Briscoe (Mirror's Edge) in collaboration with the original creator, Dan Pinchbeck.

The re-make features a completely new environment that pushes the Source engine into uncharted territory; a re-orchestrated soundtrack by composer Jessica Curry, new areas to explore and an expanded story.

Originally developed at the University of Portsmouth (UK), Dear Esther has gained a cult following and international critical acclaim since it's first release in 2007. Briscoe and Pinchbeck first showed the rebuilt mod to Valve in 2009 and they were impressed enough to grant a Source license for a full independent release.

Pinchbeck said "We're hugely excited about the license. The new version of Dear Esther will take the game to a completely new level and we're confident it will be one of the stand-out indie titles of 2011. Once again, this shows that games are probably the most exciting and innovative medium on the planet right now, and we're really happy to be right at the cutting edge of that."

"For everyone who played the original mod, we can promise a totally new experience that will keep the soul of the original whilst pushing the game to a completely different level. For people who have never experienced Dear Esther, get ready for a game unlike anything you've ever played. In 2011, we're going to answer once and for all the question of whether games can be art."

So in Steam this game just updated. Nothing change (only update) and it's cost $10 when I can download non-updated for free? Nice.

Thank you for a pointless comment.