Any good reason why? Taking last season out of the picture he hit 0.260+ in 3 of the previous 4 seasons.
Sure if you throw out his bad years he is a .260 hitter but you can't just throw out the bad years. .260 is what he hits when everything goes right for him but he is getting older, the power is slipping some, he has moved to DH which hurts production some, he is in a tougher league and his skills against LHP have slipped even more. I would think you have to project him more in the .230--.240 range and consider .260 the upside at this point.
Over the past 5 seasons he has hit .241.
Over the past 4 seasons he has hit .235
Last year he didn't manage .200.
Even ignoring last year he showed declining skills in 2010. He walked less and made even less contact that year which was a harbinger for bad things for 2011, this is a player who had a terrible season and will almost certainly rebound but it is also a player who is in a decline phase of his career as well. He has old player skills and is a big guy and those are the types who tend to crash and burn at a pretty young age. He probably has a couple more years of life in him but my guess would be by 2015 Dunn is a bench bat.